Go to Standard Cases and Pallets from the Window menu, then select the Pallets tab.
To add a new pallet, select one from the list and then use the Duplicate option on the Edit menu. This will create a new pallet at the bottom of the list.
Select the new pallet, double-click it and enter the appropriate values. On Units column type in for inches or mm for milimeters.
To remove an item from the list, use the Delete or Backspace key on your keyboard.