what hansbert posted above is quite accurate. When I went last year, the office was located in the rear of the Junta Central Electoral. We just asked and people pointed us on our way. It took all of 20-30 minutes. Really well organized.
Just be nice and patient, smile, and you'll be done in no time.
HB
This is the office that issues the C?dula for Foreigners, of course.
Dominicanos can use any office anywhere in the country..