That error suggests that there might be an OpenNI version mis-match. The version of OpenNI in the Greenfoot instructions is a little old now (I hope to bring it up to date with OpenNI soon) -- if you have also installed OpenNI for any other reason (e.g. for linking with Scratch, or if you installed OpenNI direct from their website rather than using the versions linked to on the Greenfoot website) you may have ended up with two different versions on your PC. I'd suggest uninstalling OpenNI (it lives in OpenNI in Program Files, I believe, unless it was put somewhere else -- search your system for OpenNI.dll if stuck) and installing again using the versions linked to on the Greenfoot Kinect page.

Thanks very much, I'll try that. Clare