You can add existing syndicated accounts either in the normal manner (http://www.livejournal.com/support/faqbrowse.bml?faqid=61) or by visiting http://www.livejournal.com/syn/. This page lists the syndicated accounts that you are currently watching. From this page you can remove these accounts from your Friends list, or add new ones from the list of popular syndicated accounts shown immediately below.
If the feed you are interested in already exists as a syndicated account, but is not in the list of popular feeds, you can use the form at the bottom of the page to add it to your friends list. In this case, you must enter the feed URL.
If you are creating a new syndicated account, you will need to fill out the form at the bottom of http://www.livejournal.com/syn/. You must enter the correct URL of the site's RSS or Atom feed. You cannot enter the URL of a regular HTML page on the site. You may be able to learn the URL of a site's RSS or Atom feed by contacting the webmaster or support service of the site. Users with free accounts cannot create new feeds.
If the feed URL you enter does not exist on LiveJournal, you will be prompted for a username. Insert some stuff about usernames being unique, unchangeable, and etc, or don't. Whatever.
Once you have entered your desired username, or if the feed URL already existed as a syndicated account, you will be taken to a page where you can add the syndicated account to your friends list, as well as your custom friends groups. Insert FAQ about /friends/add.bml, if such a thing exists.
Once you have created a new feed, it may take up to several days for posts to begin appearing. If you have added it to your friends list, there will typically be a large number of entries that appear at once on your Friends page. This is a one-time effect upon account creation.
General information about syndication on LiveJournal can be found at http://www.livejournal.com/support/faqbrowse.bml?faqid=178.
*shrugs* there you go :)