mapsource won't recognise gps

[crackandthump]

I've two gps's, a nuvi and zumo.
I unlocked both to use extra mapping, but now my mapsource programme won't recognise either.
Keeps asking me to change the units interface setting to "Garmin or Host".
I'm guessing that is a function performed via the unit not the PC but can't figure out how.

All suggestions gratefully accepted.

Mark