The reference is to the program's popularity within a specific category on download.com. Thus, that particular program would have had the 90th most downloads from download.com users since Cnet began logging download counts several years ago. It's just a way to put the "total downloads" number into perspective.
As to accessing the forums, there is no direct link from most download.com pages. There are, however, links to the forums in the left-hand navigation column on cnet.com as well as the tabs across the top of Cnet Reviews. You can also bookmark forums.cnet.com for your convenience.
Hope this helps,
I noticed the following in one of the download pages...
"# Product Ranking: #90 in Math Software" I noticed the same #90 on several other downloads in Math Software as well, so what does #90 mean?
Also, how do you quickly get from the download site to the forums? I couldn't find a link anywhere.