You have to have the Garmin 705 driver on you PC before SportTracks recognizes it. I couldn't find it on the Garmin site (WTF?) but found it by accident on ... I think it was BikeRouteToaster or MapMyRide - either way, I think I tried to download a route onto the 705, and it asked me if I wanted to install the driver, and then sent me to the download page.
My Sporttracks download from the 705 takes about 3x as long as the download from the 305 - anyone else experience this? Maybe I have the wrong driver?