If DNRGPS doesn’t find a GPS when it launches, and you do the Find GPS command, there is a GPS_Get error message (“System – Process must exit…”). The Montana I was using had the Garmin interface set to Serial.

To find the GPS you have to exit the software. As an aside to this, when you relaunch the software, dbl-clicking on the shortcut does not work. You have to do a rt-clk/Open or launch via the Start menu, where DNRGPS shows up as a recently-opened program.