It's ages since I used Tomtom but in the past .itn files were 'itinerary' files, and consisted of a list of waypoints. I can't remember where on the device they were located, but it isn't difficult to figure out - just create an itinerary on the device itself using its tools, save it (I think you can give it a name), then connect the device to a PC or Mac and search/browse through the folders on the device memory until you find the .itn file. If you've been sent an .itn file by someone you then just copy it to that folder in the device memory. It should then appear in the list of itineraries stored on the device. Tyre might also open .itn files but I'm not sure that Tyre works on a Mac.