I have the TomTom 3rd edition as well. Off-Road navigator will not work that well on NavCore7.xx. TomTom did not release an SDK for this software version, so when you first load it up, the screen will be completely garbled. You can do an quick alteration to the configuration file that will make the screen readable, and somewhat useable. But the interface is kinda slow, and if ever you need to enter data via a keyboard, the screen will garble again.
If you do choose to use the "Browse Map" function, the easiest way would be to set the coordinates of the geocache you are hunting as a favourite. This will show up as a star on the map, instead of fussing around moving the cursor. Be warned though, when you are within about 50m from a road, the unit will snap your location to it. I haven't found a way to turn this off.
Also, having felt the beefiness of a dedicated hand held GPSr, the TomTom feels really cheap, and fragile. I am 100% with this while the unit is mounted nicely on my dash, but walking around in the woods with the unit in my pocket? Not so much.
My plan is to pick up a reasonably priced Garmin unit for out of car use. Good luck!