I have tried entering only bin, only a26, both bin and a26. Nothing works (except a handful of ROMs). It looks like most of the ROMs I have are bin, and there are some a26. I'm not sure why ini was in that field. I had been using z26, and I couldn't get the keyboard controls configured, so that's why I'm trying to use Stella.
Are the keyboard controls easy to configure with MESS? I would give it a shot if I knew I could configure the controls...