I had to make an account to test this, but I was sure this is how this was:
Your ingame username is the exact same as your username. Whatever you used to sign up on this forum, is what you need to use in the game, with the same password.
So whatever your forum Username is, THAT is the in game Username.
Example: Let's say my Username is superawesomesauce for the forum, but the Nickname is Fluffy. You can't sign in with Fluffy in the game, you HAVE to sign in with superawesomesauce. Your password should match the in game password.
Difference between Username and Nickname: Your Username CAN NOT be changed. Your Nickname can. That's why in game, you use the Username.
Sorry if you've tried this but.. since I don't know exactly what you're doing, I gotta cover all the bases. Internet tech support is like that. xD
If that doesn't work, I suggest creating a new account.
EDIT: Ah, yes, thank you, Ruby. I couldn't think of a better term, but I knew there was one. Now I feel kind of silly. /mildly edits post.