Hi Ron,
The touch plate information in Members Documents is here.
http://www.machsupport.com/forum/index.php/topic,10088.0.htmlThe way I have done it is shown (second half of page) here.
http://www.cooperman.talktalk.net/files/17.htmAs I understand it the fet-3 only has 4 switch inputs brought out to the terminal block, as long as you have one spare input that is not being used, you could use that as your probe input.
Failing that you will have to locate an unused LPT (parallel port) input pin (10-13 & 15) and make a soldered connection to the underside of the pcb at the 25 way connector - unless you are very good at soldering this is not to be recommended.
Take a look at the above links, any questions, ask away.
This is one of those tasks that seems incredibly difficult until you have done it and then is seems just so incredibly simple.
Tweakie.