Hi Michi,
So the hardware and host driver are working, that's a good start.
Do you get any debug messages at all when you connect the keyboard?
Can you confirm that the class driver (usbhid.dll) and registry entries are present in the NK image?
The registry entries are documented here: http://msdn.microsoft.com/en-us/library/ms894890.aspx
(I know it's for CE 5.0, but I was having trouble finding a page for WEC 7.0, and anyway they haven't changed as far as I can see)
You only need the HID entries.
Carsten