See that compass-like set of numbers in the lower right of the entity window? Those are the direction the door will open. So if your door is that blue thing in the image & you want it to slide left or right (so it moves in to the wall), click the "180" or "360" button. Then run your map & it should work.
Just as an FYI, D3 basics are the same as all the previous Quake games, so a tutorial for Quake 1 on door should work for D3, just different names & some different properties.