To add a new RSS syndicated feed to LJ (i.e. create the new community), go to http://www.livejournal.com/syn/ and enter the feed URL in the box at the bottom of the page. You can only do this if you have a paid account.
In this case, the feed I just added was partialclips, a webcomic I've been fond of for quite a while.
ETA: And it already has another reader... it hadn't been a quarter of an hour. I guess I'm not the only fan!