I would like to use my MacBook Air as a extended screen for my iMac, both are running OSX 10.9.2 and both have lightning connectors. What lead should I buy? Kind Regards
Apple does not allow Target Display Mode on anything but a few select iMac models. You can see the complete list on Apples's TDM Support Page.
There may be a third party solution but I am not aware of any.