Activity Before Modified Record Saved Workflow

 Hi All,

   I have two check boxes under activity object (Version & Final). User has an option to select any one while creating but cannot select both or cannot have both as null. I got the first half of requirement via validations.

The second part is to make the Version check box to N before a record was saved, so that when user saves the record...I can give him an alert to choose between version and final.

I created a workflow with the following conditions but for some reason it doesnt seem to work.

Object:Activity

Trigger Event: Before Modified record saved