On the Montana 600,
If your at the location you want to set as a waypoint just press "Mark Waypoint".
To add another location, go "Where to" > "Find waypoint" (or add another) > "Co-Ordinates" > "Enter location". To enter letters use the up/down arrows. Press the tick to save it.
To go back to your waypoints go "Waypoint Manager".
Also you can use the map,
touch the map at the location where you want to go.
A big pin appears on the map with the tip of the pin at the location you indicated, you can move the map to adjust it. By the pin will be a box with the distance to and bearing from where you are to the point indicated. Touch the screen inside the box and a "GoTo" button appears, touch the button and the GPS will put a pink line on the map follow it.