Hi mamenewb100,
UME (MESS) only reports one resolution per system. Most systems are multi-resolution so this is a bit of a problem for SwitchRes. Fortunately GroovyMAME supports dynamic video mode change, so although not all resolutions are reported by the XML, GroovyUME can switch to them on real time when a certain game requests them. The problem is, however, that VMMaker can't create all of them beforehand because they're not reported in the XML, so chances are some of these resolutions are not available. The best solution if you're going to do a serious use of GroovyUME is to use magic resolutions. This will allow GroovyUME to virtually create any imaginable resolution on real time, so you don't have to care any more whether a certain resolutions is available or not.
Anyway, if you want to force GroovyMAME into picking a resolution, you must use the whole format: width x height @ refresh, e.g. 256x240@60
For the refresh, make sure to use the one reported by ArcadeOSD for that resolution.