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My Twitter username is @JohnDoe, why can't I use that?

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.

The "Find My Twitter ID" utility doesn't work, how do I find my Twitter ID?

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:

https://twitter.com/statuses/user_timeline/00000000.rss

Where the 0's appear is your Twitter ID.