You also have to make sure you place it in the correct folder on the NAV and if you have a memory card installed and you are storing routes onto it, then it will need to have the correct folder name structure too.
I gave up on Basecamp so I do not know what folder it defaults to when it saves them to your NAV but you may want to check and make sure they are where the NAV looks for them because it will not hunt for them - they have to be in a specific folder:
Internal Storage/GPX/?.gpx
Memory Card/Garmin/GPX/?.gpx
(?.gpx are your individual gpx route files)
This is how they are on my NAV 5 and I would imagine the NAV 6 is the same.
When you IMPORT using TRIP PLANNER this is where it looks for the gpx files; If they are not there, then you will not be able to import.