It took me a little while to figure this out too:
From the main screen on your Explorist GC, select Waypoints.
Press Menu button and scroll down to Add Waypoint.
Scroll down to Coordinates and enter them manually.
Go find your next stage!
Thanks so much! That was extremely helpful! I was debating on whether or not I needed to go get a new GPS, but now I don't. Thanks so much!