Quickplay's Efinder now includes all MESS emulators (even some which don't work in MESS yet (they are noted as such and included as one day they will work))
So just run an Efind making sure your mess emulator is included in the search path
Get ready for it to find A LOT of emulators though: this is often because MESS needs a particular call to each emulator depending on what your rom is - is ifs a tape image for c64 we must call mess.exe c64 -cass %ROMNAME% but if its a disk call we must call mess.exe c64 -disk %ROMNAME%
Now you can select the MESS emulators found to call your rom in quickplay, and you suddenly have a really good idea of just how vast the mess project is!
You can use Tempest's MESS magician if you have mess softlist roms - please see that in the tools section - amongst other things, MESS magician can change the command line run for each rom so that softlist roms are called in exactly the way MESS wants them (this often doesn't matter too much tho, just occasionally) - with softlists you call mess but then just the name of the game eg: mess.exe c64 -cass %SANXION% - quickplay achives this by calling "%ROMMAME%" instead of "%ROM%" (note the "M" - its not a spelling mistake)
So the command line for mess is
[mess.exe or mess64.exe] [system] [media type] [rom location] [and i stuck -now at the end but I don't think you need to really]
and for softlists its
[mess.exe or mess64.exe] [system] [media type] [name of rom] [and i stuck -now at the end but I don't think you need to really]