HOWEVER: note that you HAVE to download a PE map that is no newer than 0.7 alpha, since the menu and map formats were changed a bit, so that my testing showed that recent versions of Minecraft PE maps such as 0.14 DO NOT work
SPOILER FOR INSTRUCTIONS:
1. Go to a Mac/Windows computer to download and unzip a MCPE world (0.5-0.7)
2. take a USB or other removable media and transfer the map to your USB then plug it into your RPI
3. open file manager and make sure hidden folders/files are on in the View scrolldown
4. inside your "pi" folder by default, there should be a .Minecraft folder, press on it (if its not there, you haven't yet launched minecraft on your pi)
5. you will see one folder inside there and press on it, then the next folder, then the next, until you get to folder(s) of your saves, they are called "world" by default however, you can change the name of your saves and this will be reflected in MCPI saves
6.drag your save from your USB removable media drive with the rest of the minecraft saves.
7. Note that sometimes you will have to drag different parts of the saves that you downloaded, make sure that the folder you are dragging into the MCPI saves directory ONLY has .dat files in it, if you have more than .dat files in it, it most likely wont show up. if you have other than .dat files inside the save, then you probably downloaded a MCPE newer than 0.7. Sometimes, you may be able to open newer than 0.7 saves but they will have large chunks of the map missing or it will simply be an empty void world where you cant fall down.
8. If your save is inbetween MCPE versions 0.5-0.7, and it still has some chunks missing , you might of put the wrong folder inside, revise the files inside your save; or simply log out and log back in and that will solve most invisibility glitches as long as you are playing the right version(s)