how do i make a door dodad open?

Place a location on top of it, and run a Set Doodad State for the Doodad. Set (or was it Enable?) opens the gate, Clear (or was it Disable?) closes it, while Toggle sets it to the opposite state.

Keep in mind also that the location doesn't need to be statically placed on top of it, and can be set to the center location of a unit or otherwise. So long as the location is over the door by any means it will work.

Elaborating a bit further:

The trigger action "Set doodad state" will raise or lower the door depending on whether the doodad is disabled or enabled, using a location that covers at least some of the door doodad as outlined by inferno. You then need to have some conditions for the trigger to fire that action, such as a separate location that is in front of the door with a "player brings at least 1 men to [front of door location]" condition, so when they move men in front of the door, it opens. Or if you want to have some sort of computer locking system like in the Brood War campaigns where you bring marines to a 'computer console' then you can do that too (eg, you could open 10 doors on the map all at the same instant if you wanted to).

thanks guys