I am trying to add new games, but I can't seem to see the Tiny Arcade as an existing device when I attach by USB into my iMac. Is the USB strictly for charging and the only way to install games is by loading them onto a micro SD card then plugging the SD card into the arcade for installation? Or is the problem the fact that I am using an iMac instead of a PC?
Thanks in advance!

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