I just tried this so I can confirm it works under Windows (not sure on the Mac).
Plug in your GPS and a USB stick.
Create a folder called garmin on the USB.
Copy any maps (they end in an .img extension) from the GPS to the USB/garmin folder.
Fire up Basecamp. It finds the USB and the maps.
My maps are unlocked, so YMMV.
Some non garmin maps may not display this way. In which case try using: http://www.javawa.nl/jdm_en.html