Upload gravatars.php to the /wp-content/plugins/ directory
Activate the plugin through the 'Plugins' menu in WordPress
Go to the Presentation menu to change your settings.
Set your default gravatar
If you want to use the Profile Widget, you HAVE to set the USER ID and choose to use the widget, before placing the widget in the sidebar! If you don't do this, you will most likely get an mySQL error!