Installing and activating Points of Sale add-on is not enough to manage your products in your Points of Sales. You need some initial configuration:

• Go to Products tab and click on Products
• Click on the desired product to go to its configuration page
• In the Product data area select the Variable product type
• In the Attributes tab add an attribute with the following parameters:
  • Name: POS
  • Values: 0
  • Check Visible on product page
  • Check Used for variation
• In the Variations tab select Add variation and click on Go button
• Select Available for POS in the select box of the new vreated variation
• Click on save changes

Now you can create your points of sale.