Application Store cannot sign in
I was never able to sign into the Application Store with my Macbook Air. I have reinstalled the operating system, redone the authorizations in iTunes, and nothing. I do not have an antivirus installed and I do not have third party applications like Little Snitch. I have taken the Macbook Air to an Apple Store in Johannesburg and after two days they gave up and said that they were going to research a solution, but they never got back to me, and this was six months ago. I have no idea what else to do. And neither do the Apple "Geniuses" I consulted. Please help.
MacBook Air, Mac OS X (10.7.4)