Haha, I must be clairvoyant to have know the name of that file.

Seriously, gmapsupp.img is the ONLY file name that older Garmin models (like the 60csx) will recognize. You should be good to go. Just be certain to format your card as MSDOS FAT 32 (if needed), then create a Garmin folder and drop the file inside.
Indrid, I don't think Basecamp will access this kind of file, unless it's an official Garmin map. The developers recently commented in Garmin's forums that this behavior is intentional and is their way of avoiding any licensing issues.
If you do have a Garmin map file, such as a pre-loaded data card with their 24k Topo, you can use it in any USB card reader on your computer. No need to use it in the GPS itself. In fact, if you have a pre-loaded City Navigator map file on a Nuvi, you can copy it (gmapprom.img) to a USB thumbdrive and also copy the GarminDevice.xml file to the drive. Now Basecamp will treat that USB thumb drive just like the GPS and allow you access to the map. This is a good trick for people with older USB 1.0 interfaces, or if you don't want to connect the GPS everytime you need the map on the computer.
FWIW, I am a Mac user from the very beginning, starting with a "Fat Mac" (with whopping 512K of memory) back in 1985. I never used Windows at all until I got serious about mapmaking. The tools just aren't there on the Mac, and I don't see that changing anytime soon unfortunately. At least Garmin is making a nice effort to get up to speed on MacOSX, so it's not much of an issue for normal users these days.