Well, I'm quite new to mamewah myself, but I would suggest deleting your files folder in your mamewah directory, and starting over from scratch. Deleting this folder will mean you have to re-enter any settings you've created.
But once you've deleted it (or just move it) open mamewah, and it will create a fresh set of files. From there, make sure that you point it at the proper mame executable. Then set your list settings to ROM folder only, and do a refresh. This much should work for sure. If it doesn't you're pointing it at the wrong location for mame.
Once you have this working, make sure mame.dat is in your mame directory, switch your list settings to dat file vs rom folder, and then refresh again.
If that doesn't work, you might have to find someone more familiar with mamewah than myself.