In the Roleplaying Planning & Discussion forum, what you see is what you get. It's a place for aspiring GMs to garner interest in an idea, and gather a player base once it becomes apparent there is enough interest to warrant beginning a new plot.
You can find a
list of active plots on the wiki, as well as relevant information on each such as the posting frequency, links to their information pages, their current GM, number of players, and anything else noteworthy about them at first glance. That's your best place to start if you're looking to join an ongoing plot.
As Syaoran said, the Open RP forum is a "fast and loose" area for roleplay, where the requirements for joining are much more lenient. It's considered good form to ask the original poster of an RP before just jumping in (unless he or she specifically stated it is open for anyone to join), but aside from that and a generous helping of good sense and manners, you can basically do as you wish there. Because of this, though, the actions in that forum are considered either non-canon or "quasi-canon" unless/until they are officially approved as part of SARP lore.
That said, especially after your initial character submission, you should probably put forth some effort in creating an equally interesting and memorable addition to the fray. As others said before me, if your character is a boring cardboard cut-out, you're just playing filler material, an extra taking up space. Take the time to contribute something that others will want to interact with and get to know. I've seen a number of players take weeks or even months to submit their first character while they take in the vast amount of information available on the wiki. I'm not saying you
have take that long, but your contributions to the RP should make it clear that you at least tried.
On a more pointed note, after following some of your activities thus far on this site, you've made it fairly apparent you're only interested in 18+ plots. If that's your fancy, then that's fine, but I will tell you that you're unfairly limiting your search. There are a number of fantastic plots out there that don't fall under that umbrella, and you're missing out if you're not even bothering to look at them. A lot of effort has gone into many of them, and they are full of vibrant, interesting characters (PCs and NPCs alike) created by a wide variety of creative intellects and very talented writers.
It's worth noting that just because a plot is not 18+ does not mean it's G-rated Disney material either, if that is your concern. It only means that X-rated, egregiously graphic material is not permitted, but most anything that you'd expect to see in most mainstream films not specifically targeted at children 8 and under is probably fair game. However, if you are only seeking to get involved in "adults only" roleplay, I frankly believe you're looking in the wrong place. :/
Don't be discouraged, mistakes are perfectly normal. I'm not saying any of this to be antagonistic or unfriendly - I'm saying it so that you can re-evaluate what it is you're hoping to gain from playing here, and adjust your approach so that you can get the most out of your experience. Your fellow players will thank you for it as well.