When creating or editing a Business Rule, in some instances the script editor on the "Advanced" tab is missing. All that is displayed is the conditions text box.
The script window is missing on the "Advanced" tab if on the "Actions" the "Abort action" checkbox is ticked.
If the "Abort action" checkbox is unticked on the "Actions" tab then the script window becomes visible on the "Advanced" tab.
The Abort action signifies that no more actions are to take place, including scripts. If you want to use a script and Abort action unselect the checkbox on the "Actions" tab and incorporate the abort action within the script.