Whenever you get to the Citadel you’ll need to ask permission to be trained like the guys at the GNR said. Once you do that you just need to talk to the guy who does the training and you can start using it.
Well, I did warn you! Sorry for spoiling it though. I’ll make the warning more…clear.
At the moment I’m in a bit of a conundrum. I have the side quest at the Tenpenney towers (if you’ve been there) and both sides are pretty morally dubious so I’m trying to decide how I should handle it.