You need to download the .zip file and extract it to a separate directory (don’t install over your existing copy of Safari). To launch it, run the run-nightly-webkit.cmd file. Initially you’ll see nothing different, but if you load up a web page and right-click then you’ll see a new ‘inspect element’ option. Select this and you get a new window with the inspector in it.
To dock the inspector in the main Safari window, click the right-hand of the two buttons in the bottom-left of the new window.
Initially you’re presented with a DOM inspector, allowing you to traverse the structure of the page, inspect individual elements, view CSS rules and so forth.
It’s not quite as powerful as Firebug (e.g. you can’t edit HTML or CSS inline) and it’s not entirely stable (this is, after all, a nightly build). But it’s an impressive tool nonetheless.