I had a lot of trouble getting mine to work right; this should clear things up. I used this probe (http://goo.gl/WsFJGQ) - it is a LJ12A3-4-Z/BY. You can buy one from Printrbot, but they are much more expensive there. You will need to regulate the signal to 5V, but the probe will need at least 6V. I plugged the probe's power into the main input on RAMPS and used two 10k resistors to regulate the voltage as shown in the diagram above. This video (by Thomas Sanladerer) will further explain how to get things working: https://www.youtube.com/watch?v=EcGFLwj0pnA. Here is a mount for a Greg's extruder: http://www.thingiverse.com/thing:491105 You can also reference firmware configured for an inductive proximity sensor here: https://github.com/quillford/Marlin/blob/Marlin_v1/Marlin/Configuration.h.