Turn it off and on .   A simple quit that would have to be written into the game would be to reset the tinyduino, but I can't see an easy way to have the loader do this - I think IRQs would be trashed when the menu jumps to the reset code of the game it loads, but I'm not sure...

On and off is easy ;-)

What OS version are you running?  If on 7/8 you will need to install the drivers: https://tinycircuits.com/blogs/learn/171313479-tinyscreen-windows-7-8-driver-installation

Did the TinyScreen+ addin get loaded successfully?  (Sorry, I know you said you installed as per instructions, but it's two steps to install the addin which almost caught me out)

You need to remove the SD card, put the files on the card using a reader on the mac, then reinsert the card.  It doesn't appear as a drive when you plug the TinyArcade into a PC, or Mac, or anything.

HTH

The loader looks for a non-empty memory cell at a certain location (APP_START_ADDRESS+4), and if it finds it, it jumps to there otherwise it pops the "No game loaded." message.

I haven't found a way to load to a specific address via Arduino, nor the actual binaries.  Hopefully TinyCircuits can help here?