1. Make sure you've signed up for Gravatar (log-in to their site)
2. Avatar -> Use Gravatar
3. Enter e-mail address (case sensitive to what you entered on Gravatar! You might want to check!)
4. Test
5. Save
If you are getting no image after test, make sure you've selected an image on Gravatar.
If you are told you have an invalid e-mail address, make sure it matches EXACTLY how it is on the Gravatar site.
*misread OP*
How much time have you given it? It might take time to update. Make sure it has updated on the Gravatar site.