Can you put those into Basecamp?
No, you cannot put those into BaseCamp. I tried and I couldn't find a way to do it. From what I can see the only maps that will work are the ones you get on the dvd's from Garmin. At least that's how I remember it.
Those files from GPSFILEDEPOT, should come in ".TGZ" format for Mac. Just download, and 'run' that and it will install into MapInstall and RoadTrip, the newer released version of BaseCamp. Which you can download free from Garmin, under Mac version. Then open MapInstall or RoadTrip and it will show in upper right corner of screen on one, and upper left corner on the other. Oh, you need a GPS connected and turned on to get MapInstall to run. RoadTrip is OK, by itself.
I've used GPSFileDepot for Washington State 24K maps (250MB download), and NW States Topos, just fine. Gives 40 foot contours.