Hey, you'll need to use sidequest to actually install the game on your quest. Copying the .apk isn't enough.
On the PC version of sidequest, connect your quest and then hit the "install from apk" option in the top menu. Game will install, you'll get a notification then you can unplug. Mapper will be found in the "unknown sources" category of the games menu.
Let me know if that makes sense.