Safari 5 has rolled out with extensions. Before installing it, you need to enable it in your Safari browser. To enable extensions, first click on Safari gear icon given on the top right. Now click on the “Preferences”.
On the Advanced menu, click on Advanced tab. Now check the box next to “Show Develop menu in menu bar”.
Now go back to Safari browser. You’ll find a “Develop” menu in the menu bar. Click on Develop menu to expand it. Now click on “Enable Extensions”.
That’s how you can enable extension for Safari 5. After enabling extensions you can download and install new extensions. Unlike Google Chrome and Firefox there is no official extension gallery for Safari but hope they will introduce it soon. For the time being you can use this unofficial Safari extension gallery to download Safari extensions.