The install instructions are here and you may purchase an install if you think that would be better
http://www.photopost.com/installphp.html
Given your questions I think you might want to view some examples to see how things work. The gallery is a separate application and in a separate section of your site it does not appear on your forum as in part of your forum code
There are blocks to place photos and the photo cats on your forum home
http://www.photopost.com/action/index.php?cat_id=5