Where should I connect the ground wire in a 30amp enclosed switch?
Well everything but the 50 amp plug will be ok, putting a 50 amp plug on a 30 amp circuit Would violate code per table 210.21.B.3
When you connect in the box the ground attaches to the box also.
Normally a green 10-32 self forming screw fits right in one of those punched holes below the switch block. Some have a ground symbol above them.
There should be a flash guard and a cover over the contacts it is kinda important and should be replaced (the flash guard these switches are known to throw molten metal).