With your Montana have a look at the model number. If it is the 650T it would have come preinstalled with Topo maps on the device memory. If it is the 650 model it didn't come with preinstalled Topos.
What Mark is suggesting is a good place to start. Check that the Topo map is enabled.
The Montana can be used as an automotive GPS with spoken route directions so the street maps are enabled for routing.
One other thing to check is what "profile" you are in. From recollection the automotive profile enabled the street maps and disabled the Topo maps and set it in landscape mode etc.
Another thing you can do is use javawa device manager
http://www.javawa.nl/jdm_en.html. once installed, connect your Montana to the PC and javawa will tell you what maps are installed on both the Montana and the Montana's external memory card (if present).