It must extend ObjectReference, otherwise it will handle the OnOpen() event as a non-native event, and in this case will do nothing when the door is open. Your problem is likely the fact that you haven't supplied any parameters to the OnOpen event, and as you can see http://www.creationkit.com/OnOpen_-_ObjectReference, you have to provide the event with an ObjectReference. So you would have, for example:
Scriptname EnableEnemyOnOpen extends ObjectReferenceActor Property myEnemy AutoEvent OnOpen(ObjectReference akActionRef)myEnemy.enable()endEvent
Keep in mind that akActionRef isn't the door itself, it's whatever activated the door. In this case, you probably wouldn't have to worry about doing anything with akActionRef, since the player would likely be the only one opening the door.