To follow up on GeoMojoGirl's note (we both had the same problems that day), the specifics were:
Garmin Oregon 400t
The Wherigo info button gave the following:
Garmin Player v2.50
Wherigo Engine v2.11
At the time, these were the latest versions (and as of today still are). The frustrating thing is that the owner told us that the Wherigo we were trying had been tested with the Pocket PC version and a Garmin Colorado.
BriarBoy