In most cases it's easiest to just skim the bed using the machine, but since my bed is aluminium I want to make sure it's as close to 'flat' as possible so that I can remove the minimum amount of material when I skim it. If what you're probing is aluminium, then since it's a conductor you can just put a metal bar in the spindle with a wire connected to it then connect the other wire to what you're probing (i.e. the bed in this case). Connect those to one of the inputs on your breakout board, configure it as the probe input in mach3 and away you go...A quick Google search will find you the probing code.