Not working for me either. Joomla 1.7, installed, enabled, set userid to 42. on frontend, user a customer's username and my admin password and I get "incorrect username or password. Please try again." what am I missing?

Clearly you are making a mistake somewhere. I have tested the plugin myself, it works without problems on Joomla 1.7.

Perhaps your admin user id is not 42? Perhaps the account you tried to log into was an admin account? Perhaps you are using a login module that is not the normal Joomla one and does not use the normal Joomla authentication system?

Perhaps your admin groups are not the normal Joomla ones? By default the plugin assumes groups 7 and 8 are the admin groups, if your site uses a different scheme you will need to set the correct groups, it is important for the security of your site that this is set correctly, you do this on the advanced options.