We had a plugin here on this page that allowed users of WordPressMU to run multiple instances of mu within one install. This was called multisite. When 3.0 came out, the terminology was changed. And with it, the plugin was updated and moved to a new home. You can now find the Networks Plus plugin at WP eBooks.
This plugin does not enable the network feature. It allows *multiple* networks in one install, after you’ve set multisite up according to the codex instructions.
If you’re looking for domain mapping, you can find it here: Professional Domain mapping.