Sorry about the late response. I was on the road, on the way home from class. I kinda left you hanging, didn't I?
So, to join another role play, first you need to open up its page. I'll assume you're familiar with it since you've created one already, but if not, I will be glad to explain it in great detail. Once you've got it loaded up, click the Characters tab. You'll see a bunch of characters that are already a part of the RP, but if you scroll down you'll find the Character Application. You need to fill out all the fields here (except the "avatar" picture is not necessary) and then click Submit at the bottom of the page.
Alternatively, from the Introduction tab you can find a Submit a New Character button along the right-hand side. If you click that, you'll be taken straight to the application.
Once you've filled out the sheet and pressed Submit, you'll be immediately sent a private message with a link to your character. Now, you just need to wait for the GM of the RP to accept it, and then you can start posting. You'll know when you've been accepted if your character appears in the Characters tab. Generally, the GM will inform you if you've been accepted, but if not that is how you'll check.
Does that help you out? Let me know if there's anything I can explain further.
You're probably fine in this regard, but if you're not sure how to fill out the character application, take a gander at
this video. (YouTube link)