Click on the door object in question, and find the Animator bar (should be a tab on the right). Right click the animation list (at the top) and select Add Animation. You will now have a new blank animation which you can add frames to (right click the frame area at the bottom and select Add Frame). To rename the animation, click on it, then see the properties in the property bar. Call it something like 'Open'. Be sure to click the Angle listed under Open and set it to not loop, if you want the door to stay open! Then in events you can change animation to 'Open', or 'Default' if it's closed.
If you have any further questions on this please ask