Cubase should give you the option to either record as a mono track or a stereo one. The graphical representation of the track will probably also reflect this (a mono track will show one waveform, a stereo one should show a pair). I use SONAR, and it's pretty easy. Check the help file for how to do it, it's probably a little "button" on the track control area.
Tip: When recording an instrument that has a mono signal (bass, guitars that you're not tracking effects for, vocals) it's a very good idea to track in mono. Tracking mono instruments in "stereo" (dual mono) doesn't offer any benefits - and it offers several drawbacks (larger audio files, more difficult to effectively pan the track, heavier processor load on real-time effects, phase issues, etc.) When mixing, you can still run the mono track through an effects processor that creates stereo effects.
Obviously, keyboards that have different Left and Right channel signals should generally be tracked in stereo to retain that. If you must record a keyboard in mono, find out how you can sum the L/R outs to a mono feed; if you just record one or the other, you can lose some of the important sonic info.
Hope that all helps, I rambled on a bit.
