How do you add data from a supplemental table to a subject line?

I want to dynamically insert text from a supplemental table into the subject.  I've tried writing RPL in the subject line, and I've tried using the Dynamic Rules builder in the campaign message designer.  In both situations, the values I'm trying to pull from the supplemental table show up as either null or 0.

Furthermore, when I add the supplemental table as a data source to the workbook and de-select the option "Data source is used only as a lookup table", I get the error message: "[SUPPLEMENTAL TABLE NAME] has no extraction key that matches fully to the list."

But my supplemental table does have a data extraction key identified, and that field's values match one of the primary key fields in my contacts list.  However, the field name for the extraction key in the sup table is different from the corresponding field in the contacts list.