The JoiPlay savefiles are hidden in the root of the gamefolder. You need a file browser that can show you hidden files or just download savefiles from here and extract it into the gamefolder and add/overwrite them.
You can download the latest Joiplay-savegames from AnonimeGamer and extract the zip-file in the gamefolder. You can extract the zip-file in Android or with Windows/Linux onto your Phone/Tablet.