Google AdSense Site Authentication

I ran into this issue just now and finally discovered the values that worked for me

Restricted Directory or URL: http://www.[yoursite.com]
Authentication URL: http://www.[yoursite.com]/user
Authentication method: POST
1st Attribute: name
1st Value: [yourUsername]

2nd Attribute: pass
2nd Value: [yourPassword]

3rd Attribute: form_id
3rd Value: user_login

4th Attribute: op
4th Value: Log in

If the values I provided do not work for your site you should be able to determine the right values from your page source, within the code for the login box. Hope this helps!