How do you associate a MPN to a Supplier item number that is associated to an internal item?

Summary: In PIM I have created an internal item number to use for inventory. I associated the item to a Trading Partner Item and would like to associate the trading partners item to a manufacture's part number. After creating the item number in PIM, I went to Relationships, Trading Partner Items and added the Trading Partner Type Supplier to add the supplier and supplier item number. A second section opens to show Linked Manufacturer Part Numbers but when I try to add it, it does not find the MPN. I can add the MPN as a Trading Partner Type but this not what I want, I want the trading partner relationship to be the Supplier with the link to the MPN.