You must start training at the Brotherhood of Steel Citadel to wear Power Armor of any variant.
That requires a special perk called "Power Armor training." At the beginning of the "Picking Up the Trail" main story-line quest, talk to Elder Lyons (not Sentinel Lyons) in the laboratory section of the Brotherhood of Steel Citadel, and he will grant you training permission. Now head back out into the main complex of the Citadel, where Brotherhood initiates may be found practicing their live arms training. Talk to Paladin Gunny and he will now give you Power Armor training.