What do you have in Manage | Database | Relationships for this file?
This drop down lists the names of each "box" in Manage | Database | Relationship. If it's not listed in the drop down, then there's no "table occurrence" box on the relationships tab. This is the case even if the table you wish to specify is defined in a different file. In those cases, you'd click the button at far left, bottom of the relationships tab to add a new occurrence, but then you'd use the Data Source drop down to select or add the needed data source table. In your case, you'll likely need to select the add option so that you can find and open the file where you've defined the table from which you want to look up values.