Do you have the Garmin AC/PC adaptor cable? It appears that this thing plugs into the wall for power, and has a DB9 connector for serial data.
If you are trying to get by without purchasing a cable, the connector pinout is on page 83 of the owners manual, available here:
One more thing: I think most electronics use 4800 baud as the default speed. Most PC's serial ports are set up at at least 9600 baud, so they won't talk without some adjustments.