I got my 60CS week ago and it CAN connect to MacGPS through serial cable and Keyspan USB serial adapter. I have PBG4 12 inch and iMac and it works on both.
Points to consider: You need the newest driver (download from www.keyspan.com). I don't have different type of adapter, so can't comment on it.
Make sure su set the right communication protocol in your 60C. For downloading/uploading of waypoints you need GARMIN protocol (Main Menu -> Setup->Interface->Serial Data Format->GARMIN), for real time display of data from the unit you need to change it to NMEA In/NMEA Out).
I played with this last time for the first time. If you have more questions, let me know.
Edited: Few typos