
You can add this if you right click on the title area of Safari and scrolling down to "Customize Toolbar...". Simply drag this icon to your browser window next to another icon of your choosing and click "Done".
If you go to Twitter, log on to your account then click this icon you can select around the input field (most likely it will snap to the suggested element to mask) then click "Add" in the purple bar at the top. It is now conveniently placed in your Dashboard.
Now that you have it in your Dashboard its as easy as opening Dashboard and adding a post to Twitter.
It's that easy!
Enjoy!