Reviewer Property

Hi Guys

I am a complete newbie to BPC, I have got a BPC 7.5 for Microsoft system using BusinessObjects authentication. I have added Reviewer to the Category Dimension however when I try to process the dimension I get the error "Your Excel sheet or source table must include the REVIEWER property." I have added a user to the Reviewer column in the Dimension spreadsheet but I still get the issue. What am I doing wrong ?