An important aspect to note at this point is that in sometimes Webtrends takes a long time to number crunch its reports -- long after it reports that data analysis has completed, reports can still be empty.
Be patient. Build the reports, reanalyse the data, then wait (often serveral hours) for the reports to be populated. If you're using Webtrends On Demand, likely you'll be used to the waiting for the data analysis job, so will already be conditioned.
How to add a custom report
Check the short video below for details on adding a custom report