So, simple problem that I am having.  Hopefully I am missing something obvious and this is easily fixed.  I searched the documentation, as well as did a cursory search of these forums here without seeing this question pop up.

I can't seem to roll the camera.  I see the shortcut, and button for it - which is greyed out mysteriously.  I see in the documentation a description of how to use it, but the shortcuts it describes don't line up with what my software seems to be doing.  I'd like to be able to press V and roll the camera - I'm wondering why I can't do this.  There is nothing in the documentation about switching between different view modes or camera control methods - I've looked, though maybe I've missed it.  
 
So, how can I roll the camera?