To manually assign a permission to a sproc in SSMS 12...

1. right-click on the stored procedure in the Object Explorer and pick "Properties" from the menu which appears
2. pick "Permissions" from the menu at the upper right
3. then click the "Search..." button to bring up the "Select Users or Roles" dialog box
4. use the "Browse..." button to get what you want and then close out the dialog box with: "OK"
5. click the "Grant" checkbox for "Execute" for your line item and then click "OK"