In system prefs, under sharing, on the MacBook Air, enable Screen Sharing and write down the VNC address that is displayed. Then on your Mac in the Finder Menu bar, go to 'Connect to Server' and type in "vnc://xx.x.xxx".
You'll need the user name and password of the computer you are trying to control.
A more simple way to do this would be to share screens via iChat, assuming you're both running Leopard.