I find turn by turn directions a bit pants for MTBing so all I wanted was to see the route trace on the map so I could follow it. In the end this is the way I found to do it (from memory so forgive me if I get some details wrong);
I don't use any of the course stuff (.tcx, .crs), I convert to .GPX files using this software and then copy/paste to the Garmin. Shows up in "saved rides". Select the ride you want to follow and (iirc) click the "map setup" button. You can then choose to display the route on your background map in a number of colours. Chose a colour, click "show on map" and back out of the menu system to the map view. Voila, your route is marked on your map in red/yellow/whatever colour) for you to follow