Before you can apply any rules to a proof in Pandora you first need to set the proof's conclusion.

You can do this by clicking on the No More Given button, next to the input field, below the toolbar (preferably after you have added your given lines to the proof). Then enter the conclusion formula is in Input field next to the button and click on add.

For information on the syntax used in Pandora to enter formula's see Logic Sytax

Note: You can not apply rules to your proof until you have set it's conclusion.