Find this feature at http://www.facebook.com/twitter or
Under Edit Page>Editing>Edit settings>Resources
Allow facebook to post to Twitter (if you are already logged in to Twitter, you will only have to click on Authorize App.
To edit settings or remove the link:
Edit settings (use check boxes to select/unselect)
Unlink to Twitter to remove process.
If the Unlink to Twitter doesn't work, you can also try remove the authorization via Twitter.