If you've simply downloaded the TT map that should be installed onto your GPS it will be a single .img file. Basecamp cannot read that off your PC, although it can if you copy it onto a SD card or memory stick.
If you want to install the maps onto your hard drive for Basecamp to use you need to select the mapset designed for use with mapsource/basecamp on the TT site. It's a zip file and inside the zip file are instructions for how to install it onto your PC.
