How do I find a break in my ground wire?
I would start by turning off the circuit, pulling out the outlet and ensuring the ground wire is connected.

If the wire is connected, but still showing as not connected, I would get a cable tracker and follow the wire.  The cable tracker has a sender that sends a signal through the wire, and the receiver listens for this signal.  You can use it to follow cables through the wall and ceiling.  When you get to the break, you should loose the signal.

Here is the cable tracker I have.  http://www.harborfreight.com/cable-tracker-94181.html 

It would be unusual for the break to happen outside of a box/junction.