To access the forums, you need to start up the game  Tricky Truck before entering the forums.
This is to protect against ugly spam bots.

More info: When you run the game, the high score server will log your IP address and add to the list of allowed IPs of the forums.
The reason for this protection is that spam bots have been registering in the forum, even passing CAPTCHA text reading test, and making posts and text in their introduction about offtopic material, such as enlargement and Russian pr0n.

Contact: here