Glenn,
No real surprise here as I have not validated XMLdb for V5 or DNN. XMLdb is based on V3 of DNN and thus is rather old.
However, this does give me the push to get it to work with V5.0. I will create a version compatable for V5.0 in the near future -- stay tuned.
A possible alternative is the Reports module -- its has some similar functionality in the version released with DNN V5.
If you are in a rush, let me know.
Paul.