I don't have an RM100 , but the basic MIDI setup begins with the foot controller ( midi mate ) on the same channel. Your RM100 ( if new) is probably set to channel 16( or it may be channel 1) . Connect the foot controller to the midi "in " on the back of your amp, set the Midi Mate to send on channel 16, then press one of the Midi Mate bank switches, then also press and hold the channel switching button on the RM100 to select the channel, hold it for 2-3 seconds, next press another Midi Mate bank switch , and repeat by selecting the next channel ( module ) on the RM100 and hold for 2-3 seconds. If your send /receive channels are correct you should be able to switch back and forth between the 2 MTS modules using the Midimate now. You could also just connect the Midi Mate to the Xpression midi input and make sure they are working properly, then midi thru from the Xpression to the RM100, but again the channels need to be set. It's easier to change the send /receive channels on the Xpression and Midi Mate, than mess with the dip switches in the RM100.
You can also check the Egnater site ( www.egnater.com ) for a manual describing the MOD50 or MOD 100 , I believe they share the same midi functions as the RM100, but I'm not positive.
Found the manual for the Mod 50- http://www.egnater.com/manuals/mod50revA.pdf
Hope this helps,
Howard