From the manual:
"There is a REVERB BYPASS jack on the Rear Panel (Bottom Rear Of Combo Chassis) that accepts a standard ¼ “ phono plug and by connecting a simple tip-to-ground type latching switch to this jack the REVERB can be triggered on and off. The EFFECTS LOOP must still be in the circuit for the REVERB to work and triggering this REVERB jack with the FX & REV switch in BYPASS will not turn the REVERB On."
There are lots of simple tip-to-ground type latching switches you can use, so you're not limited to the Mesa switch. Mesa wants $50 + shipping for theirs:
http://mesa.stores.yahoo.net/fcr.html