Take the adjuster off! [take off the black part with the arm and cable stop] YOu only need to have the main part of the brake, the bolt that hold it in place and the V shaped cable yoke/hanger - then the outer cable stops on the other side of the seattube, but you need a threaded barrel adjuster to sit in the hole in the seattube. So it goes - brake lever - outer cable - barrel adjuster in seatube - then cable clamps to the V yoke just behind the brake. Yo can get a barrel adjuster anywhere - new school bikes use them for the rear ad990 set up in the same way.
cheers!
steve