What OS are you running on. ?
I've seen a problem which we haven't been able to resolve, and it only seem to happen on Datacenter.
Here is a test you can try, because if this works the library should work.
From the start menu run wbemTest , click connect (top right) and change root\cimV2 to root\virtualization and click connect. Then click enum classes type MSVM_computerSystem and click OK.
This should give you a list of VMs. If you don't WMI is not working properly on the machine and the library won't work.
If this does give you a list of VMs, then we can see if we can figure out what's broken.