can I activate the weapon master power from the war domain while I´m in a barbarian rage?
Rage (Ex): "While in rage, a barbarian cannot use any Charisma-, Dexterity-, or Intelligence-based skills (except Acrobatics, Fly, Intimidate, and Ride) or any ability that requires patience or concentration."
Weapon Master (Su): "At 8th level, as a swift action, you gain the use of one combat feat for a number of rounds per day equal to your cleric level. These rounds do not need to be consecutive and you can change the feat chosen each time you use this ability. You must meet the prerequisites to use this feat."
Supernatural Abilities (Su): "Using a supernatural ability is usually a standard action (unless defined otherwise by the ability's description). Its use cannot be disrupted, does not require concentration, and does not provoke attacks of opportunity."
It's not a skill, and it doesn't require concentration. The only question is if it requires patience.
Ask your DM. Personally, I say it works fine.