Here's how to do it:
First, click the appropriate link to make a comment. (On here, it's "Say Something" but the default is "Post a new comment".) Click the OpenID radio button. Type "openid.aol.com/username" into the box, where username is your username on AOL/AIM, and without the quotes, of course. (It's probably best to click the "Log In" toggle box, too.) An AOL page should pop up after you type in your comment and hit submit. Your screen name should already be inputted. Type your password and put a check in the "Remember this site" box. Click the "Sign In" button. It should take you back to LJ, and your comment should be made, like this.
After you've actually have an OpenID account on here, the login page is at http://www.livejournal.com/openid/. You can upload some pictures, change settings, etc.
63 Million more (potential) users is good.
Edit: People have asked me what the use for this is, since anonymous commenting is already enabled. Well, the more effective use for this is to add people as your friends and they can read friends-only posts by you. Heck, they can add RSS feeds and people to their friends list, then use their friends page as a big feed aggregator.