Hi NormB,
If you are lucky and the tool is already at GND potential then you only need the one lead, connected to the copper plate. This lead is connected to any spare input and configured within Mach as the Probe. There are numerous macro's for the auto tool zero button and you just choose which one best suits you.
If it is any help, the way I have implemented it is shown in the bottom half of this page.
http://www.cooperman.talktalk.net/files/17.htmTweakie.