SONAR has the annoying habit of assuming you want a stereo track by default. The key is to set the track interleave to mono before recording, and then select a mono input from your interface. It's worth the time to set up a track template that way, as that will make it easier for future recordings.
The problem with stereo tracks is that the concept of "panning" has no meaning for them. A stereo track is essentially two mono tracks hard-panned left and right. Even though there is still a pan control, it's really just a balance knob like you have on your stereo. It makes one side louder than the other but does not shift the stereo image.
If you want to keep a track in stereo but be able to "pan" it, use the Channel Tools plugin instead of the pan slider. This works like the dual-pan controls you're used to in ProTools.