Gary,
If you are referring to a simplisitic smoke 'leaking' through the 'cracks' of a door, initially you need to create two or more separate pressure zones between the two areas separated by the door. Within the zones you can specify an leakage area.
You can then create a 'air leak' surface and assign this to an obstruction. Also, assign the specific zones to the 'air leak' surface.
This should create the simple leakage path for smoke through a door.
However, please refer to the user guide which provides more details.