I encountered one problem on my MacBook Pro and it left me quite depressed. Each time I wanted to login and check for the new Apps I couldn’t. This was quite the issue because I am awaiting for the new update of Mac OS X Lion, which brings some good new changes to Apple computers and bridges the gap between iOS and Mac OS.

So after browsing quite a while I found a good solution that can help you in three easy steps.
Step #1 – Open Terminal
Step #2 – Paste this
rm -rf ~/Library/Preferences/com.apple.appstore.plist
rm -rf ~/Library/Preferences/com.apple.storeagent.plist
rm -rf ~/Library/Preferences/ByHost/*
Step #3 – Press Enter.
Viola, your problems with logging into App Store are solved! You can now purchase apps and keep them up to date all the time. And I wish you successful update to Lion OS X!
Disclaimer: If this didn’t help you, please leave a comment so I can find a more viable solution for you, this solved my login issues on Mac, it doesn’t mean it will solve your as well.
If this did help you, get in touch with me and start following me on Twitter or subscribe on my mailing list.