Hey Griphook, If you bought a MP2 new, as I did back in the day... it came with the MXC pedal pack which included 7 pin midi cable, MXC midi pedal, one expression pedal and one quad switch. The expression pedal and quad switch connect to the MXC via standard stereo (R/T/S) jack leads, apart from that it's really solid, the expression pedal is no different to any CC type expression pedal, it has a pot inside which is turned up/down by the pedal movement. It includes a "soft" switch when you press at "full on" position (like a wah that tuns on/off). You can assign the pedal and the switch to any parameter in a CC compatible gadget you want to sweep (vol, wah, an eq freq, delay time etc...) or toggle with the switch (eg turn wah on/off)
The quad switch (again really solid) is 4 buttons (same sort as the MXC buttons) with a LED under each button. You can assign each button to a CC controllable parameter you want to turn on/off. The buttons can be individually set to latch or unlatched. Latch means when you press it it stays pressed (on or off). Unlatched means its on as long as you hold the button down and reverts to off when you stop pressing on it.
The quad switch is probably a bit more special than the expression pedal but all the groovy midi pedals being discussed here lately do much the same. The CC number assignment is done in the MXC which has 4 stereo jack inputs, 2 for pedals and 2 for quad switches (so it matters which jack you plug into (pedal 1/2, switch 1/2) as this decides their controller numbers (which you can change BTW via the pedals function commands). The MXC sends the midi change data so anything reading the midi stream can respond to it (assign controller number(s) to parameter(s) in the CC compatible device).
The quad switch was designed to give you more real time control over some MP2 on/off parameters (FX loop, tremolo, tuner mute etc) but as Dante likes to do, you can also use them to control other CC devices.
http://adadepot.com/manuals/MXC_Owners_Manual.pdf