Always check the 
LuaDocs before you try this kind of thing.
Now onto the problem.
MovableObject:IsActor takes no arguments.  It just tells you whether it is an actor or not.  What you want to check is the MovableObject's ClassName.  That's what it is defined as in the INI file (e.g. ACDropShip, ACRocket, AHuman).  So, a working version of the code would look like this:
Code:
for actor in MovableMan.Actors do
       if actor.ClassName ~= "ADoor" and actor.ClassName ~= "ACDropShip" then
             self.active = true;
       end
end
Also, in case you're wondering, "~=" is an operator that checks for inequality, or, in plain English, checks if something is not equal.  It's the opposite of "==".