In theory, all you should need to do is go here:
http://www.teambuick.com/forums/forum.php
and click on the name of the forum or sub-forum where you want to put your post. A list of the threads in that forum will open. The "Post New Thread" button is at the top of the list on the left side.
However, I have noticed that a few forums like Classified, Buick Garage and Events (there may be others) do not have that button, and I don't know the reason for this. Which forum are you wanting to post in?
Ray