This worked for me on a different Garmin GPS so it might work for you on the Montana.
Hey! I got it.
I had both My Trails and mitopo11 on my etrex30 and copied those two files to my laptop then to my GPSMAP 78SC (Garmin directory). I then disabled all maps on the device excep My Trails and mitopo11. I opened a track and WOW there it was on mitopo11!

So...if you have acess to copies of the two files already on a garmin device, you can copy and paste into your GPSMAP 78SC.
Hey! I got it.



So...if you have acess to copies of the two files already on a garmin device, you can copy and paste into your GPSMAP 78SC.