In the arming screen, notice the boxes labeled "Empty" at the top of the screen. Click these boxes to remove all ordinance off the aircraft. Once done, click just to the right of the rightmost box, outside the grid. An "X" will appear. Click on the boxes in positions two and eight of the B-57 to load nuclear weapons in any mission.
To change aircraft, the new airplane must be available during the current mission. Enter one of the case-sensitive pilot callsigns on the pilot screen. The <xx id> part of the code refers to the mission ID of the indicated aircraft. For example, if there are four F-117s in the current mission, the value of <xx id> would be 00 to 03. After activating the code, your Hornet will be under CPU control and fly at a set altitude above the original starting location.