I assume you are doing this by connecting the 60csx directly to the Mac with a USB cable? The 60csx has the really old USB 1.0 interface, maybe that just isn't compatible? I have one but never use it anymore.
But maps are stored on the microSD card on the 60csx, not on the GPS itself. If your mac has an SD card slot, pul the card and use that instead. Otherwise, use a USB SD card reader, that is a much better solution since it's so much faster. They should be cheap if you don't already have one. Get one with USB 3 and it will be really fast.
Of course, there could be something else odd happening, but if you're connecting directly to the GPS itself, I'll bet that is the problem.