You have probably noticed that PowerPrep does not display the answers for the questions at the end of tests. D:
Luckily, we have explanation videos for many of the questions from ETS PowerPrep and the Official Guide, including the PDF of the paper-based test (because it's a static document that's easier to reference). You can access them here: Magoosh GRE Official Guide Solutions. You might need to scroll to the bottom and follow links to "other official GRE material." I would also suggest checking out this blog post on the overlap (there's a lot) between the Official Guide, PowerPrep, and practice PDFs: ETS Practice Tests: Powerprep II and the Official Guide Book and CD.
Whew! That's a process. With that in mind, the easiest way to find our answers/explanations is just via Google. If you search site:magoosh.com "[the first few words from the question]", you'll find our explanation video near the top of the results, if we have one.
You should also know that our solutions are not complete—we don't have videos for every single question. If you are a premium member, feel free to reach out to them at email@example.com if you can't find the solutions you're looking for. If you are not a premium member, there are usually forums and discussion sites you can find through Google that will also have good resources and people with answers! :)