HELP! RewriteRule gives 404 Not Found

Can anyone help me? I've never been able to get the Apache rewrite
rule to work. This is what my .htaccess file looks like.

RewriteEngine On
RewriteRule ^moo$ index.php

I've tried this on my localmachine, my mac machine, on a third party
linux hosting site, but it NEVER WORKS!!! I keep getting a 404 not
found!

What do I neeed to do???????