Should there be an air outlet in an attic with a whole house fan?
The concept of a whole house fan is two fold.  One is to bring in the cool evening air, the second is to flush out the hot air that is trapped in the attic.  Your attic should already be vented enough to allow the incoming air a place to push the hot air out.  If you where to make an opening for the fan to vent it, the cool air will just travel straight to it and not cool the attic as well.