Same problem with all the GArmin units. The unit requires a voltage on one of the pins to allow you to reflash the EEPROM and the USB adaptor is not capable of doing it (the USB port in native mode is). The serial adaptor is actually a "PDA" adaptor and so only works on a limited range of devices.
Garmin have addressed the problem on the new 296 and that comes with a USB cable as standard.
I have had to keep an old PC around with a serial port just for doing updates as all of my laptops and PC are legacy free and only have USB ports!