If you go into the Google Music player on a desktop browser, you can go to the auto playlist called "Free and purchased". From there you can Ctrl/Cmd-click all of the songs that you want to download. If you point at one of those songs, to the right of the "Name" field - just before where it says "Time" - a small triangle-shaped control shows up. Click that and you will see a menu with a "Save to computer" option. You can download a song that you purchase twice, though the Music Manager (strongly recommended) does not count against that total. (The Music Manager on the Mac at the moment does not download songs yet; that feature is coming.)