Clarify what you want to do, are you trying to leverage information about your mame games to categorize them in different lists (i.e. Fighter, Run N Gun, SHMUP, Beat em up, Puzzle, etc.) that roll up underneath a tree layout? or do you have multiple game-lists (defined it Mala) that you want displayed when that games list is active?
If you meant the latter then all you have to do is edit your mala layout so that on of the text fields has the string of code I mentioned a few posts up.
If you mean the former then it's a bit more involved, but can be done. See tree layouts in the WIKI.