In addition to what Roman has advised. When you set up these multiple ROM paths in MAME/MAME32 use a relative path instead of the full rom path.

Instead of:

C:\games\arcade\Multiple Arcade Machine Emulator\ROMs\NeoGEO

Use just 'ROMs\NeoGEO'

In MAME32, you can edit the ROM paths from the GUI, by single clicking (may have to do the single click twice) on the path that you want to edit. Then remove everything before ROMs.

Also, MAME32 is limited to 20 directory entries, and there are at least 23 systems. To fix this, you would need to edit the src/ui/directories.c file to change MAX_DIRS.

NOTE: This only fixes the number of directories problem in the MAME32 sources. You also need to increase the allocated rom_path size in the MAME sources. If you use relative paths, then the problem with rom_path alocation size shouldn't affect you.