I've completed the main story line...but have yet to find or figure out how to attain Brotherhood training for the T-51
power armor......

Why's this in DLC forums?

Anyway, go to Hidden Valley, complete BOS' quests, an you'll get it.
Alternatively, complete Gannon's quest. (I'm not 100% sure this gives you the training though.)
If you've killed Gannon or something like that and killed/destroyed BOS then you can't learn it from what I know.

Here the wikia entry for it: http://fallout.wikia.com/wiki/Power_Armor_Training

Well, first stand on one foot, jump up and down, spin, single "Jingle jangle jingle" while wearing Motorrunner's helmet and eat your goddang vegetables.


Obviously I'm kidding, do what gabriel77dan said.