The brake release is on the brake lever. There's a small cylinder in the horizontal plane on the outside (front facing) edge of the actual lever above the pivot. Snap that little guy to the other side and the levers will allow the brakes to open up. Do the opposite (might have to compress the lever) to close the brakes.
Keep in mind that sometimes a builder will setup with brakes with the brake lever release open. If snapping the little guy doesn't seem to allow the brakes to open up this might be the issue... You'll have to release the cable from the brake and readjust them with the release closed.