Hi samw, I thik there are a few points where your config could have gone wrong, so check them one by one.
1. After changing the es_systems.cfg file you have to do a restart, so that emulationstation loads the new paths.
2. Be carful, because MAME has more than one emulator and you should check that the file extension is correct (.zip).
3. It seems pretty odd that your path has MAME_0.151_ROMs twice. Please check it again. If the path points to a directory which doesn´t exist, you won´t see anything in emulationstation.
You could try another approach, instead of changing the path in es_systems.cfg you could just symlink to the network mounted directory from the original directory.
For this you´ll have to go to your mame emulator folder (ex. /home/pi/RetroPie/roms/mame-advmame) and ln -s /media/consoles/MAME_0.151_ROMs/MAME_0.151_ROMs
if that is indeed your correct path.