Boy, that thing won't cut a very nice circle...
Not sure if you have Baldor or MSI motors/drives. Either way, they are on the axis cards on the rack in the middle of the cabinet. It has the large capacitor on it with a resistor on top of the cap.
Each board has small pots along the edge. Tach/gain/BAL etc. They should be labeled on the board itself. You need to slowly adjust balance till you get the 0 or so on axis stand still.
Your drives are really in need of a good tuning though. Some old Tree employees bought the leftover rights, they should have the drive tuning procedure. Can't remember who they were. You will need a good multimeter to do it.
Maybe start here
ZPS-CNC USA, Inc.
I think Bob shows up here from time to time, but he is about the best with Tree stuff.