If you are having trouble replying to a topic or starting a new thread make sure that you have clicked the activation link in the email that was sent to you when registered. By default the system continues to let you browse the forum like any guest would but activation is necessary to post/reply.
In case you did not receive a validation email, you can click the "Resend Validation" button on the top right of the page (Once logged in) to send another one. Please note that for some users it may end up in the spam folder so make sure that it did not get routed there.
This is done to keep the forums clean and combat spam.