You want to first create a relationship between the contact entity and the opportunity entity. This will create the lookup attribute you are trying to create.
The article below will walk you through the process:
http://rc.crm.dynamics.com/rc/regcont/en_us/op/articles/addlookup.aspx
Brian Begley XRM Services LLC bbegley@xrmservices.com http://www.xrmservices.com This posting is provided "AS IS" with no warranties, and confers no rights.