I've tried road trip and base camp with my zumo 665.
Nothing will load a map from my macpro to my garmin.
I see by reading this forum that i am in good company.
Useless zumo 665.............$700
I do not have a Mac but the Manual page 31 says it will connect to Mac but you might have to map the garmin drive if it is not visible as a mounted drive.
Then page 31 says you must make and save your custom route as a gpx file. Then you must copy and paste that gpx file into the mounted drive or the mounted SD card.
Often times the necessary steps involved to get these devices to do what you want them to do is confusing.
For starters, I suggest you make your route by saving some waypoints at places you want to go. Then let BaseCamp "auto route" between them. If you do not like the route then right click the route > Properties > change from Motorcycle to Scooter or whatever and then re-calculate the route and see what happens.
You might have to add an extra waypoint here and there to make the software go where you want.
When you get what you want then save the route, in gpx, to a folder you can find. Then copy and paste the gpx file into your Zumo drive or the SD card in your Zumo (if you have one).
ALSO, you must change the routing preferences in your Zumo to match the settings you used in BaseCamp to make the route. Manual page 9.
OK, disclaimer. I don't own a Zumo 6XX, POS in my opinion. But, this might work. Do not load the route to the Zumo internal memory, export the routes to the memory card and then import them individually to the unit. IIRC, this gets around the Zumo recalculating each route (which it's designed to do) when loaded. Good luck.
