MAMEWAH has a "default" list for every emulator that you can not add anything to. This gamelist is always the first one when you bring up the menu. Are you asure you weren't trying to add something to this list?
Yes. I had added a new list called "Katherine's Favorites" (Katherine is my daughter) and tried adding games to it: no joy. Later, I changed the default "Filtered" list to a "Favorites" list and set it to the type of list that automatically shows the most popular games, At some point after this, as I was going to delete the Katherine's Favorites list, I tried it again and it worked. I don't know if changing Filtered to Favorites had anything to do with it, or if perhaps I just didn't have the control scheme down yet and was cancelling the Add to List rather than selecting it, but it does work now.
Hey... I just realized that something else I did between the time it didn't work and the time it did is regenerate the main list. Do you have to regenerate the list after creating a new list? I'll bet that's it... I'll try it again when I get home.
--Chris