Not sure if this is related to the problems that either sonnyangell or jaedigga are having, but...
In response to JDFan's posting regarding default rom-file types in Maximus, not everyone necessarily has their NES rom files in a compressed format. As such, if you're trying to load, say, .NES files directly, you would have to add the .NES extension to the aforementioned scan/preferences setup section of Maximus. Essentially, whatever format/container your own actual roms are in, you must have that particular file extension explicitly specified in that console's settings section within MA.
If I'm not mistaken, any files whose extensions are not specified in MA's settings will not even show up on the list of roms when MA is running. Now, if games are showing up in the list but cannot launch within MA, that's another problem entirely. If roms will load directly with the emulator or through Windows' file browser, then perhaps MA has some bad command line parameters associated with the emulator in that console's settings.
I haven't used an SNES emulator lately, but Nestopia Undead Edition seems to work quite well for NES roms through Maximus, with very minimal setup needed.