Forum registration is a multi-step process.
1) When you register, the IP address you sign up from is checked to see if it has spam associated with it.
2) Your email is checked against known spammers.
3) Your email must exist!
4) You are sent an email with a link to activate your account. Click on the link!
5) A moderator, or admin must approve you.
What can go wrong...
You already have an account using the same email. Get a new password sent to you.
The email bounced because your email account doesn't exist. Can't help you.
The email generated an auto-reply that indicates I must sign up to send you email.
Ain't gonna happen.
You selected a username that is lacking any discernable meaning.
You selected a password that will put our web site, and Forum at risk.