Plucker desktop doesn't support RSS (currently, anyway). You'll have to look for another way to use RSS feeds with Plucker.
If you have access to a PHP-capable web server, and know a bit of PHP, you could try to format it into basic HTML using
Magpie RSS, or something similar, and then grab that with Plucker. Just be sure to use the caching options, as most websites don't like having their RSS feeds hit more than once every 30 minutes (Slashdot, in particular, is very strict about this.)