I know this is a very old thread, but since it took me forever to find out how this works, I'll post anyway.
To relate records from two tables that already have a relationship class defined, do this:
- Open the table view for both featureclasses / tables
- Select the records you want to relate
- On the Edit tab on the ribbon, click the Attributes button to open the Attributes pane. The top part contains the selected records.
- Expand either one of the records, so that the name of the relationship class becomes visible
- Right click on the relationship class, and select Add Selected to Relationship
