Hi.

I have purchased the MGMAT study guides ... I have also registered onto the ManhattanGMAT site, and have access to all my practice tests and question banks etc.
However, whenever I enter the forum, I enter I am entered as a "guest" ...

When accessing the Forums, you actually do not need to log in. As you create your post, you can manually change your username to whatever you prefer (a box very similar to "subject" will appear at the top.)