I've opened the Login Window Settings program on Debian Squeeze. It's asking me for an admin password. I haven't changed my password from the default (raspberry). I tried entering raspberry but it gets rejected.
This worked for me :
open LXTERMINAL
type sudo passwd root
you will then be asked for a root password - type in a password and press enter
you will then be asked to repeat the password
then it will say password updated
now use your new admin password to run your program.