If you don't see the sign in sheet > Mac App Store: Sign in sheet does not appear, or does not accept typed text
And disable any security software such as anti virus before trying to access the App Store.
Make sure your Mac meets Lion's requirements:
- Mac computer with an Intel Core 2 Duo, Core i3, Core i5, Core i7, or Xeon processor
- 2GB of memory
- OSX v10.6.6 or later (V10.6.8 recommended)
- 7GB of available spare
Please read before upgrading to Lion > Lion upgrade questions: Apple Support Communities
Installing the Mac OS X 10.6.8 Update Combo reinstalls the App Store for you which may help.
Restart your Mac after the combo is installed. It's ok to do this even though you are already running v10.6.8.
edited by: cs