1. First u have to make a brush to look like a door.
2. Then another brush with the same height,same thiknes, but not as long as the door. ==ATENTIONM==ust be the ORIGIN brush.
___________
top view: |_|________|
^ ^
origin you choos
brush brush
ATENTION==origin brush must be inside the door brush!!!!
3. Group them bouth.
4. Click TIE TO ENTITY and choose func_door_rotating.
VOILA!!! You got your door.
And put here your e-mail adress I'll send the tutorial if u don't understand what I said.