You can associate Products in your Markets with UPCs from the VMS or from the Kiosk itself. It is generally recommended that you scan and associate them through the kiosk so that the UPC is guaranteed to be recognized, but both methods can be found here.
Adding a UPC Through the Kiosk
At the kiosk, you will need to tap My Wallet. Tap the green 
Items without a UPC are highlighted in red.
Search for the product if needed and tap the Add UPC button underneath the product thumbnail. When prompted, scan the product UPC and hit Assign. A popup will follow to let you know if the UPC was successfully assigned.
If the UPC was not assigned, a popup will appear detailing why, as seen below.
If you encounter any issues assigning UPCs to products, you can contact Parlevel Technical Support to help you in getting the correct UPC assigned to your products.
Adding a UPC Through the VMS
In the VMS, go to Markets > Product UPCs. All of your products with a UPC assigned already will appear here.
Click the + Add New UPC's button.
In the following pop-up modal, find the appropriate product using the Product field, and then scan or manually enter the UPC(s) in the UPCs field. Any previously-entered Saved or Duplicate UPC's will be shown here for the selected product.
Click Save when finished.