Just paste this URL to your browser address:
http://api.twitter.com/users/show/[your Twitter username].json
be sure to change the Twitter username accordingly (without the [ and ]).
When browser prompt for download, just open it with a notepad. You will see a bunch of text.
Find something like this;
331383958 is the ID.
Now if you would like to get the RSS for that user, use this:
(again, without the [ and ]).