NOTE: This applies only to Safari 5.0.0, not newer version of Safari like 5.0.1. In 5.0.1 and later, the Extensions pane of Safari’s preferences should be enabled by default.
For those who haven’t already:
1) Make sure ‘Show Develop menu in menu bar’ is checked in the Advanced pane of Safari’s preferences.
2) Make sure ‘Enable Extensions’ is checked in Safari’s Develop menu.
3) There should now be an Extensions pane in Safari’s Preferences. Start trying out extensions! Double-click on a .safariextz file, and Safari should ask you whether you want to install it.