These instructions refer to eXodus 7.1.
eXodus (www.powerlan-usa.com/exodus.html) is the recommended X emulator for Macintoshes used at PPPL. (There are also paid-for copies of MacX around, and these may work fine.)
There are extensive instructions on setting up eXodus at http://web.gat.com/comp/exodus_support.html. Also see http://fusion.gat.com/docview/exodus.html. The instructions here do not cover various options so they are simpler.
Many X-windows applications make use of the middle and right mouse buttons. The Exodus defaults for the middle and right mouse buttons are the cmd-2 and cmd-3. These can be changed from the Settings menu (but don't change them for no reason).