It depends slightly on what device you have. If your device connects to your computer via a usb cable and shows up as a storage device, then from the page in Garmin Connect that shows a list of your activities, click the "import" link near the top right of the page. Then browse to the activity file you wish to upload. Some devices will have it in the /garmin/activities folder, others might have it elsewhere.
If your device connects via an ant stick, then you'll have to get some info from someone more familiar with that. Or check the Garmin Express forum or Ant Agent forum.
