If you, like me, have tried to install a Safari 5 extension such as Coda Notes, and were met with the following dialog:

and then spent 5 minutes looking through Safari’s Preferences window to no avail, this is how you enable Extensions on Safari 5:

  1. Open Preferences
  2. Click Advanced
  3. Click Show develop in menu bar
  4. Close and reopen Safari
  5. Click the Develop menu item
  6. Finally, click Enable Extensions

Viola.