Short version:
Go to http://www.dohamap.info/map/ and type in 'Police'. It's the last result in the list (Madinat Khalifa Traffic Department).

Long version:
Find Oryx Roundabout and head out of town on Khalifa Street. Straight over first junction and you come to TV Roundabout. Straight over that and you come to Markhiya Roundabout where you'll see a big Burger King sign. Straight over that roundabout and before you reach the next one you'll see signs to the Traffic Police via a slip road. Take the slip road and then the first right.

If you're reporting an accident then you need to drive into the courtyard immediately on the right once you turn the corner.

If you're going for anything else (i.e. registration, driving license etc) you need to park in the big car park just past the accident section and then walk to the end of the road where you'll find the main station through the barrier on the right.

Translation services/assistance can be found in the two little 'shops' on the opposite corner. They can be very helpful.