The 25-question bonus question banks are associated with the red strategy guides, so to access the SC guide, you have to have purchased the SC book somehow (either by itself or as part of a course or self-study program). Have you done that?
If so, when you log into your practice center, the default page will be the CAT page. Just under where it says "Practice Center" you should see a line of red text. These are a series of links: CAT Exams, OG Stopwatch, Question Banks, and so on. The thing you want is the "Question Banks" - click on that red link to reach that page.
If that link is missing for some reason but you do have the SC book, then contact tech support (800.576.GMAT or
techsupport@manhattangmat.com) for help.