I'm pretty sure that a lot of people have had trouble with this. After installing RealOne, they still can't play Real video streams in the Safari browser window. Sometimes Safari will shuffle the content off to RealOne, but sometimes it won't, forcing me to shuffle through the page source, get the URL for the stream and then paste it into RealOne. This gets annoying. I've never seen a default install of RealOne play video in the browser window.
So I wondered, "Why the heck didn't Real make a browser plug-in?" Well it turns out that they did; it's just not put into your ~/Library -> Internet Plug-Ins directory by default (as far as I can tell). The necessary files are stored in the RealOne application package at RealOne Player.app -> Contents -> MacOS.
Copy these files into your ~/Library -> Internet Plug-Ins directory.
Then simply re-launch Safari, and you'll be able to watch the stream in your browser window. If you still can't, try copying the Library directory inside RealOne Player.app -> Contents -> MacOS to your Internet Plug-Ins directory. In my experience though, this doesn't make a difference. Also, this may or may not work on video windows which use JavaScript controls. Real is a little flaky with those.
Mac OS X Hints
http://hints.macworld.com/article.php?story=20031023122119600