I use QMC2 under Linux, mostly because I couldn't get GMAMEUI display options to work right. I think I complied QMC2 from source since the install files kept complaining about some version error that I couldn't make sense of. Once you point it at the right locations for the mame executable and ROMs, the rest is pretty easy. But I just looked around the QMC2 settings and I cant find where the rom path is set. I think I just edited the ini file to set this when I first installed it, but that option is not in the GUI as far as I can tell, unless it's so easy to find that I can't find it.
And why is there a mp3 player built into it?
I gave up on MESS, I tried using it so I could run TI 99/4A stuff without switching over to Virtualbox to run Windows XP to run Classic99, but I could never figure out how to load and games. I got to the point where I could run TI Basic, very slowly.