Hi Damian, welcome to the board. Understanding polyamory is hard. Doing it is harder. You might be jumping the gun. To aid you, I will critique your ad, and offer some advice from my decades of experience in polyamory.
Hello everyone who m here to ask a few questions.
Do you mean, "Hello everyone! I am here to ask a few questions"? I am asking because, if you want to land a (reasonably intelligent) woman online, you need to be able to write a grammatical ad, and then be able to write messages back and forth that make sense. Otherwise, you just sound slapdash and not serious.
You see I've been putting up ads on different places like Craig's list or like a meet -up and I keep getting replies about one [night] stands or just bedroom buddies or some nonsense like that. So Id like some criticism! Help me understand what I'm doing wrong or even better just tell me if I'm just nuts! Here is the Craig's list post:
mw4w body : big status : married
age: 29
Is that your age? Or are you both 29? If you can't put it in the header, mention it in the body of the ad.
Hello,
My wife and I are looking for a female
A female? A female what? If you mean human, say woman.
a third in a stable relationship
Being "a third" doesn't sound very attractive. It sounds like she'd be the unstable one, teetering on top of the two of you, who have known each other a long time.
Ungrammatical.
If you like to go fishing, hiking, walks in the woods
hiking is a synonym for walking in the woods
or just hang out at home play games and listen to music We do too and we'd love to take you!
or just hanging out at home playing games and listening to music, great, so do we. And we'd like to do it with you!
If you and/or your wife enjoy hiking and fishing, why not join a Meetup group for those activities? Then you or your wife might end up finding a nice lady (single and open to poly, or already in a poly relationship, or relationships) on one of those group dates, that you or your wife hit it off with, and one of you might find the opportunity to ask her to go out on a real date.
We like to find new places in and out of town,
Use a period (.) here. New sentence:
we like to go to Palmer, Wasilla, Kenai, Fairbanks, Seward, all over the state!
We like to try new things, so if you're into something, we're flexible.
Of course all women are "into something." It sounds so condescending to imply she may not be "into something," but if on the off chance she happens to have hobbies or interests, you'd deign to "flex" to her interests. And consider, she might be into something your wife likes, and you don't, or vice versa. You're not a Borg. You, singular, are probably not going to "flex" to go get mani pedis or shop for bras. (If you're metrosexual, you might, just making a point here.)
We are not looking for NSA or simple flings,
Period. New sentence:
fuck buddies need not reply.
You might leave that line out. It's a bit gross.
We want something serious and meaningful.
All of us polyamorists want something meaningful and not merely sex-based. (Amory= love.) Yet, most people who respond to ads just want cheap meaningless sex. Or worse, they are so anti-social, they just want to talk dirty for wank material. OTOH, it's nice to indicate you're looking for a real relationship, but most relationships start out kind of light and easy until chemistry and compatibility is established. You gotta kiss a lot of frogettes to find a princess!
By the way, all the "we, we, we" seems very couple-centric, and probably intimidating.
I myself I like to read and write stories,
I like to read and write stories. My wife likes to (...?)
I like to hang out and watch a movie on the couch[,] usually with someone

.
The wink makes it read like you're saying "Netflix and chill," ie: fuck.
I like to go on trails, both in town and out.
You already mentioned hiking and walking.
I'm picking up photography as a hobby and I'd love for someone to experience these things with us!
You like those things. Does you wife like those things? If not, don't say "we" yet again. Say what your wife's personal interests are.
"We" again!
And you want a woman, not a female, not a girl. Immature women who are needy and un-self-confident, lacking self-awareness, and often financially challenged, and who may have unresolved issues from their childhood, tend to be attracted to the *perceived* security of dating a couple. You don't want one of those, do you? At 29, do you want to adopt an 18 year old?
who is open minded and not afraid to share it. We are an independent couple[,] both with stable full time jobs. We are both bigger people but are still very active. If you're interested we eagerly await your reply. Come play and enjoy life with us.
Please[,] if you seriously want to get to know us, tell us your favorite color in the subject line so we know you're not a robot.
All the couple-centric "we" stuff has been addressed by others. Women are not lining up to seriously date an established couple, no matter how stable, active, fun and "open minded" they are.
So, read up on the pitfalls of unicorn hunting. Check your grammar. Then don't bother with Craigslist. Determine to date separately. Join your local polyamory group. Join a Meetup group for one of your hobbies, and/or join OK Cupid (it's free). I have been there many years, and I find one good man for about every 30-50 who message me. As a bi woman, I have found one good woman (we've been together 7 years, and we date men separately, despite all the begging for threesomes we get). If your wife is bi-curious and only wants to try dating a woman, she can filter who sees her on OKC. If either of you are afraid to try dating others, if your wife is afraid to fuck a woman without you in the mix, then you're not ready for polyamory.