I followed this tutorial and got problem that certificate has expired.
I looked up in Air forum and found http://forums.adobe.com/message/2320586#2320586 which says the same problem that I got. Unfortunately, it has not been solved.
I am looking forward to your suggestion.
If you need a self signed certificate, you can create one from Flex Builder or the SDK. If you are using a commercial certificate, you might have to pay to extend it.