I can't work out how to write midi patterns that will trigger  session drummer in x3. I have loaded a kit, I can click on the drums and a single sound can be heard, I can load a pattern and that can be heard. Step sequencer in greyed out, piano roll is silent.
Thanks.

If you are not sure the Session Drummer is inserted into SONAR correctly try using one of the predefined track templates. Right-click in the track strip area and select a template from Insert From Track Template > Soft Synth Track Templates > Session Drummer 3.
 
There are a couple of videos on Drum Production using an older version of SONAR with subtitles for the X series which might help get your started http://www.cakewalk.com/CakeTV/SONARU.aspx/Master-Class
 
A forum member created a video showing how to use the PRV with Session Drummer http://www.youtube.com/watch?v=ruAnAPR6-hQ