This page still only has one validation in the Server Validations server behavior. You would have to add validation on the Model before you would be able to show a message based on that validation failure.
1) Double Click on WA Server Validations in the existing Server Behaviors window
2) Click the lightning bolt at the end of the "server variable:" line
3) Choose the form and the "Model" field and click "OK" to close the bindings panel
4) Click the plus (+) button
5) A second validation will be added to the list of validations.... click "OK"
Now you would be able to show messages based on either or both validations failing with the Show If server behavior