Trying to connect Garmin GPS to Macbook via Serial-USB adapter

I have a pretty old fashioned Garmin eTREX Vista that still uses a 9-pin serial cable to connect to computers. I bought a serial-to-USB adapter, and have tried connecting the device to both my Macbook and my Macbook running Windows XP via bootcamp. Neither system recognizes the device. I've downloaded Garmin software that's supposed to search for devices, to no avail.

Any suggestions would be mucho mucho appreciated.