You can't. DCUO is an MMO. When you purchase an add-on, it goes on your account's Services List. Not the Downloads List. That means that only the PSN account that purchased the add-on can use it. Other accounts on the same PS3 have no access. In most cases add-ons that are actual downloads, can be shared with other accounts on the same PS3; although not in absolutely all cases. It depends on what the developer of the game decided to do. If your brother wants the earth power option, then he will have to buy it with his account.