you haven't verified your email address. Within 1-30 minutes after you register an email is sent to the address provided when you registerred. Without clicking the link in this message you will not be allowed to post.
If the message has not arrived check your SPAM box, usually it gets dumped in there. If the message gets deleted... I guess you're SOL

 

apparently this needs a sticky.
if you didn't get the email go to the user CP and change your address, it'll resend it.

 

you also need to reply to a post before you can start a new thread, this is to keep the spambots away.