You first need to mark the Product so that it is available for return. To do so:
Open the Product Record
Click on the Balances button (bottom-right)
Click on the Batch Records tab at the top
In the lower half of the screen Items in selected Batch
Double click on the line where it has a Status of General Stock
In the pop up, choose the Warehouse of Damaged Stock or Goods Returns from the dropdown list
Click Save
When you go back to the Supplier Returns look up, the Product will appear against the OWN account