All I have on my sd card is a series of files and an overlays folder? When I've installed Retropie in the past there's the usual folders such as config, bios and most importantly 'retropie' inside which will be various console names ready to place roms inside them?
I've also tried inserting a usb drive with a 'retro pie' folder created on it. What's supposed to happen is that retropie sees this and creates the required directory structure on the usb stick. But this doesn't happen either?

Could someone tell me what I'm supposed to do to get my gbz to run roms?
Any advice much appreciated.