Log into Facebook, then go to http://www.facebook.com/privacy/ (or via the “Account” dropdownmenu choose “Privacy Settings”)
Click “Manage Blocking” under the part “Blocked People and Apps”.
Applications & Users you have blocked will be listed here. Simply click “Unblock” on the application or friend you want to unblock. The application will now be unblocked, so you may feel free to search for it and re-add it to your profile.
Good luck, and happy Facebooking.
Last Updated: October 2011