If you were one of them, we apologize for any inconvenience. It turns out that your school's internet access was blocking Facebook access, and Headmagnet uses the Facebook Connect feature. This caused other scripts to fail, resulting in the error message.
This is now fixed. [Updated: April 10, 2010: This still doesn't work for everyone]
You should be able to sign in now using your email/password. However, if you school or company's internet access is blocking Facebook, then you still will not be able to use the Facebook Connect feature.