Hi Murpha, I had trouble too when I first installed Mame4Droid on a tablet a few years back. It's a lost-in-translation moment: if I remember correctly, when you think you're telling it where your Mame folder is, what you're actually telling it is where you'd like it to create a Mame folder.
So I would delete the lot, then reinstall Mame4Droid, tell it where to create your Mame folder and then drop your Roms, samples etc in to the folder.
I hope that helps, and that I'm remembering correctly!