Here's a couple things to check:
Did you reply to the message the forum sent you when you registered?
When you originally registered for the forum, a message was sent to you with a link in it. By clicking the link, you should have completed your registration. If you didn't click the link, the forum is still waiting to confirm your registration. It'll know who you are, and recognize you by saying Good Day your username, but it still won't let you post.
We do this to keep from getting crawled by web bots and having spam posted.
If you can't post, but have registered, do two things. Dig around in your inbox and see if you see a message sent on the day you registered. (You can find out when you registered by looking in your Control Panel and looking at your profile) It has a link in it. Click the link.
Alternatively, notify the admins here and we'll either register you manually or send you a new link to click.
Do you have your browser security set too high?
If you're using Internet Explorer, you need to let the browser accept cookies. If the forum can't place a cookie on your machine, it won't be able to keep track of what you've seen and who you are.
Let your machine take the cookie from us. It's tasty and we don't track anything you do.