Can’t use thesaurus function in Outlook 2007

I recently upgraded from outlook 2003 to outlook 2007 when our company switched from having POP3 to Exchange. I DID NOT upgrade the rest of my office products; however, now I can’t use the Thesaurus function in Outlook – it is grayed out. Any ideas on how to fix this? Could it have to do with different versions of office products not “talking”?