You may not be able to get in, because the Olympic Games is coming, and the control is very strict. You have to find a way to contact.
Take 359 from Dongzhimen to the Capital Airport Terminal, and then walk 200 meters south. Or take the airport bus to Terminal 2, and then take the airport bus to get off at the Flying Corps. The rest depends on how you contact her.
Take bus 107 from Di 'anmen to Dongzhimen, and then take bus 359 from Dongzhimen to the Capital Airport Terminal.