When you create a Google AdSense account, a Google account is automatically created for you at the same time. With a Google account, you can access a variety of Google products and services, including Google Analytics and Webmaster Tools. Gmail is Google's free email service. You are not required to have a Gmail account to sign in to a Google AdSense account.
Instructions:
1) Open your Web browser and navigate to Google.com/adsense -- this is the Google AdSense homepage.
2) Type the email address and password associated with your Google account into the respective input fields in the login box.
3) Click "Sign in" to log in to your Google AdSense account.
4) Click on "Can't Access Your Account?" if you are having trouble logging in. The most common reason for login trouble is a forgotten or incorrectly typed password. Follow the on-screen password prompts to reset your Google Account password, if necessary, and then try logging in again.
Tips and Warnings:
If your Google account is associated with a Gmail account but you would rather use some other email address to log in to Google AdSense, open your Google account dashboard and click "Edit," next to "Email Addresses." Type the email address you want to use to log in to Google AdSense and click "Save." You can now use this email address to log in to your Google products, including AdSense.
Source - eHow
