After I enter my username and password at the login screen, the message appears "The system was unable to unlock your login keychain".
Keychains in Mac OS X are Apple's password management system. They are stored locally on the computer for each user that has logged in, and are specific to that user.
Especially after an online password change or computer swap, a keychain entry may become out of sync. If keychains are misaligned, the message shown below may pop up after entering your district username and password at the login screen:
Select the button for Create New Keychain.
You may be requested to type your password in for various systems or websites the first time the system or website is accessed after a password change. This will rebuild the keychains stored for your account on the computer.
NOTE: Remember, after changing a district password online at http://password.nsd.org, always run the patch in Self Service called 1st Aid: Fix Local Items Keychain Message and Reset Keychains. Self Service is in the Applications folder. instructions for using Self Service are in KB article #555.