If you’re having troubles signing in (the login window doesn’t appear or you can’t type anything in the input boxes) then it might be because you’re running FruitMenu or Application Enhancer. Apple has acknowledged that problem here http://support.apple.com/kb/TS3638. If you don’t want to disable FruitMenu then fcarroll suggests a workaround:
There is a relatively easy workaround for this problem. When an attempt is made to log in to the App store with Fruit Menus running, the log in window appears behind the main window instead of in front. If the main window is moved out of the way, the log in window will be visible. You can’t type into it but, using the contextual menu, the password can be pasted and the log in button can be clicked.
If you get a never ending loading screen that reads “One moment please. Connecting to the Mac App Store. Loading…” then you’ll need to delete two files and restart.
To delete the files paste the two lines below into Terminal and press enter:
I did stumble upon another very similar solution by Paul Bagwell but it deletes some extra folder. If deletion of the two files plus a restart didn’t help I’d definitely give that solution a try. If you find some other problems/solutions please post in the comments and I’ll update the post.