A trigger is a brush-based entity. Now, if I recall correctly, what you'll have to do is make a brush textured in AAATrigger texture. Place this brush on the side of the door that you want it to open from (what'll happen is that the player will hit this invisible trigger in-game and the door will open). Make sure the brush doesn't spill over the door so that it might accidently be triggered on the other side.
Select the brush and click the "ToEntity" button that's on the right hand side of the Hammer window. Select "trigger_multiple" from the list that appears. In the Target field, put the name of the door in (so name the door if you haven't: you can do this by selecting and right-clicking the door in a 2D view, scrolling down to "Properties" and given the door a name in the Name field). In the Delay before trigger field, put 0. In the Delay before reset field, put 0.
That should be it. Read our Door tutorial if you need help on making a moving door.