If you are unable to post to the forums with your newly-created account, it is likely that your account is still in the "validating" stage.
When you signed up, you were sent a validation email to the address you provided during registration. Find this email and click on the link it contains - this will immediately validate your account and you will be a full-fledged member.
If you did not receive a validation email when you signed up, send an email to firstname.lastname@example.org and we'll take care of it for you.
Note: Accounts that remain unvalidated after a certain period of time will be automatically deleted.