Tracks are generally not routeable on topo maps. I generally use waypoints and the map and eyes method as well rather than using a routeable track gpx route.
There's a few ways to create routes on basecamp.
The simplest is the route between two points. If you create more waypoints, you have a more accurate route that follows a mapped track rather than the straight line "as the crow flies" route between the waypoints.
There's a few video guide on you tube like this one (although it is a bit dated using an older version of Basecamp, the principle is the same)
https://www.youtube.com/watch?v=U75KTv-dxB0 It should be similar on a Mac as well.
I tend to create routes using waypoints from a library. Basically it involves selecting multiple waypoints and then selecting the option File/New/Route using Selected Waypoints.
But you can use the Route drawing tool as well, and draw it on the map. File/New/Route then select the New Route tool and on Windows at least, when you finish drawing your route on the map, you either press ESC or rightclick the mouse. Then on the route properties, you can either edit or delete your points, or reorder the waypoints. You can check the route by replaying it to check if you've mucked any points up.
If you have gpx track files from somewhere, you can select the option New route from track.
To join routes (on a windows machine at least), you select the first route, then hold Ctrl key down while selecting the next routes in the order you want to join them, then pick Advanced/Join the Selected Routes. If route orders get messed up, I right click on the selected routes and it gives options to Move up/Move Down/Reverse that section of the route.