You should be approved to post on the forums if you have purchased our CAT exams and still have access to them (ie, it wasn't so long ago that your access has expired). So if you are not seeing a button in those folders allowing you to post, contact our student services team for help (
gmat@manhattanprep.com, 800.576. GMAT in the US and Canada, or you can use the chat feature on our website—it's active at certain times of day).
If you haven't already, I would also recommend searching the forums for the text of the problems you want to discuss. You may find that someone else has already asked the question that you want to ask and then you can read the teacher's reply right away.