In Al Duhail

https://maps.google.com/maps/ms?msa=0&msid=203872640511665582080.0004c8b...

Once the plate is ready you should get an SMS in which you are notified to go to the above mentioned place to change your plates.

Also refer to previous threads in QL about the same topic

http://www.qatarliving.com/node/4467356 and here

http://www.qatarliving.com/answers/cars/new-car-number-plates

Regards!!