This has been discussed here awhile ago, but in relation to the GPSMap 62s which is another new Garmin model. DNR Garmin is old software that isn't compatible with the newer Garmin handhelds such as the 62s, Oregon 450 and Montana.
The problem is that it "talks" directly to the GPS using an old Garmin protocol. "Back in the day", Garmin created a proprietary protocol for exchanging data between a computer and GPS using the serial port. In other words, the computer says "hello", the GPS then responds and the computer queries what capabilities it has. It can then request that the GPS either send or receive data.
Later, Garmin started using USB to transfer data but still supported this protocol. But newer models have abandoned it completely and data is now exchanged on the file level by mounting the GPS as a USB disk. The handshaking where the GPS tells the computer its capabilities has been replaced by a file called GarminDevice.xml that describes the unit's capabilities. The actual transfer of data is accomplished by just copying files in both directions.
So that's the "why". I am not aware of any fix for this, since the new models just don't respond to the data request from the DNR Garmin program. If you must use that old software, I think all you can do is send the data to an old GPS, like your 60csx, then use Basecamp to copy it to your computer and send it to your Montana.
BTW, your Montana is also not fully compatible with Mapsource. You can use Mapsource to send data to the GPS without issue, but it will not receive all your data from the Montana. This is because the Montana and other new units store waypoints and tracks in a bunch of separate files, organized by date but Mapsource only recognizes one file for these.
Just another thought.... The arcmap suite is way above my "pay grade" so I'm not familiar with its capabilities. But do you have the option to export the data you want as a .gpx file? If so, I think that will do what you want. If you place a .gpx file in the \Garmin\GPX folder on your Montana, it will be automatically imported. But a better approach would be to open the .gpx file in Basecamp and have a look at it first. You could then send it to your Montana using "drag and drop" in Basecamp.