Downloading music directly from Google Play Music onto your phone is intended for offline listening on that device only--you won't be able to find the mp3 files easily. It's kind of a soft way of discouraging rampant file sharing. But you can download the album directly to your computer from your Google Play Music library page. Once downloaded to your computer, you can do whatever you want with it. Keep in mind Google Play Music will allow you to download a given album a total of two times, so if you do download it, keep the files safe somewhere.
If you install the Google Music Manager program on your computer, you can download music an unlimited number of times through that program--but it can be inconvenient, because you can only choose to download your entire library, or just your purchased and free music.