Cannot connect to the app store

Hi all,

My girlfriend has a macbook air, and wanted to update her computer to Lion. I had no problem doing that on my own macbook pro, but I can't do it on hers. She didn't update her softwares regularly, so at first I found myself having to update a ton softwares so I could finally install the Mac App Store. Once it finished installing though I was never able to actually use it, as when I open the application it just says: "Cannot Connect to the App Store". I looked up online and people had similar problems, and none of the suggested solutions worked:

- Starting up the computer in "Safe Mode" did not work;

- Repairing all the permissions in the Disk Utility application did not work;

- Signing out and in the Mac App Store from the App Store application did not work;

- The problem is certainly not the internet connection because I was able to update my computer in the same connection, and I can regularly access it on my computer.

Does anyone have a tip to solve this problem?

Thank you very very much in advance!