If I'm not mistaken, there were two ADB trackballs from Kensington: the earlier one had a small panel which, when prised off, would reveal a 4-5 switch bus, the configuration of which would allow you to set the trackball's button's behaviours; and the later trackball, which did not have that panel. Unfortunately, the right button on the TurboMouse is not supported in configurations of OS X above some arbitrary point - ISTR it's 10.3 or so - so you might as well let go of that immediately and move on.

However, there's an alternative or two: wire that second button so as to act as a twin for the left button; Install FinderPop and arrange for the contextual menu to appear with a mouse-down-and-hold of the left (one) button; and, both of these together.