SquirrelMail won’t authenticate anyone

I've just installed SquirrelMail by following the directions in the hint on AFP548.com... everything seemed to go fine. PHP's working okay (did a few tests), and the Squirrel login page comes up. But no matter who I tried to login, it says that the user and password is unknown. Anyone else had this issue? I would have presumed that the user and password would be the same as the Mail user and password, but it's just not working... Are there configurations that I can check to see how it checks authentication? Where should I be looking for logs that might give me a hint?