If you need to display data in the report header and those data might change while printing multiple pages (as it usually is the case when talking about document reports), you still must use the GetData/SetData concept to make it work.
But consider that SetData had to be slightly changed for NAV 2013 (RDLC 2008) - just copy it from the standard NAV 2013 reports:
- In the SetData textbox in your report header, create the data expression not under "Expression", but under "Hidden"! ("Expression" is left empty)
- In the RDLC code section, the SetData function must return value True (so that the SetData textbox itself will be hidden)