TomTom menu > Exit Application
Make sure you've exited the GPS Receiver software before launching TomTom. On the main screen upon loading up TomTom, tap the right lower corner where the GPS signal strength bars are. That should get you into the Route view. Next tap the vertical bar on the right to bring up the GPS Status. You can of course do this through the menu system as well.
Once you're in the GPS Status, tap Configure. Select Other NMEA GPS, and then select the baud rate, and then the COM port. The COM 1 option display is messed up on my screen. It shows as "Se???a?? ?a??d?? st?? COM 1:"
I don't use a serial GPS. But see this post:
http://www.aximsite.com/boards/showthread.php?t=126247