This is all extremely confusing, and it would be nice if we could take 5 extra seconds each in our responses to be clear for people who are actually trying to learn. I'm not sure what it is about these sorts of threads, but nobody is clear about anything. Is this on purpose?
If we are talking about MAME cores in Retroarch, can we be very, very clear please? I'm staring at the list right now. There are 10+ MAME cores in Retroarch, and it doesn't seem that anyone is interested in referring to them clearly. There is no "MESS" core in Retroarch. There's a "MESS 2015". Do we mean that? Does that make a difference? Does anyone know, and have they tested it, or are we just guessing? If not, which other MAME core do we mean specifically, by its specific, exact name in the core list. There is no "MAME" core. There are other options like "MAME 2003", or "HBMAME", but no "MAME" core.
I'm not able to get this system going through Retroarch either, and I have the folder named properly. I've tried booting with quite a few MAME cores with no luck.