First make sure you have the latest version of IE, java, javascript, and flash installed. Also, make sure you have no software (eg: flash-blocking utilities) that are blocking the tomtom.com website.
If the above doesn't help, try resetting MyTomtom:
- click on the myTomtom application, and go to settings-> cache, and click "empty cache", then "save settings"
- click on the myTomtom application, hold shift and click "Close", then "Exit"
- open the folder %UserProfile%\Local Settings\Application Data\Tomtom
- delete the Home 3 subfolder within that folder
- restart MyTomtom from the Start Menu, and see if it now recognizes the GO 2505.