NOTE: In order to associate an invoice to an existing PO, the PO must be associated to a vendor. Blanket or Unassociated POs cannot have an invoice associated to them. If the vendor has no POs associated to them, the drop down list will be blank.
Open the invoice you wish to associate to a purchase order. Then, click on the "Edit Invoice" button on the top right.
Enter the appropriate vendor and select the desired PO from a drop down list of POs for that vendor. Click on the "Submit" button.