Adds a link to your Twitter feed in the header of your WordPress Blog.
Because of the way that Twitter generates RSS feeds for their users. Twitter doesn't use your username, but uses a numeric ID to identify each user. "Add Twitter RSS" now includes a utility to find your Twitter ID from your Twitter Screen Name.
Go to your Twitter Home Page and look for the RSS icon for your updates on the right hand side of the page. Hover over the link and the full address shold appear in your status bar. It should appear something like this:
Where the 0's appear is your Twitter ID.