real alternative installs the real codecs needed to decode the stream plus the registry entries needed so that other programs, such as MC, can find and use them.
In laymans terms, you're installing realplayer without all the garbage that traditionally comes with full installer. All you need to do is install it, make sure MC is associated with the real file formats (tools > options > file associations) and then get MC to load the url that points to the stream and off you go.
That's the simple theory.
Sadly, it's often not as simple as all that.
Take the BBC website for example. They state that the only way to listen to their streams is by using their web-based stream player, but with a little detective work scanning the source code of the webpage it's possible to find the direct address for the stream and then enter that directly into MC (File > Open URL) and then you have BBC Radio 2 playing through MC
The best you can hope for is that you install the codecs, click the link, and MC opens and plays the thing. In my experience that's rarely the case though, and a little detective work is required more often than not.
happy hunting