Johnathan,
when you say you can't get it working in the field, I am presuming that you have it working in your office. Is that correct?
If no is the reply, Garmin devices when connected to a computer typically don't show up as a port without the use of Garmin's Spanner application. All Spanner does is create a virtual COM port, and through this the Explorer GPS code can connect to the port, receive the gps sentences and display your position.
Let me know how you get on.
Edan