Thanks to a user for reporting this
The Garmin Foretrex 201 uses a USB-to-Serial chip for it's USB adaptor which is manufactured by Prolifict (the PL2303). As with other PL2303-based solutions, a driver is needed to make it work on the Macintosh. Prolific has a driver site that contains drivers certified for use up to OS X 10.6.
We are also told that the drivers from the US GlobalSat site will also work, but those should basically be the same drivers, since it is the same USB/Serial chip.