To get it, you either have to finish the Operation:Anchorage DLC, or proceed in the main quest until you are told you can get it. This and the training will both happen in the BOS Citadel. I had a problem finding the trainer, but i think the guy was in the middle of the Citadel yard. You may need to wait a few hours if the guy isnt there. Maybe it is only offered at a certain part of the day. Well, i got it a long time ago so this may not be accurate... the answer should be on the Fallout wikia.