Here is Netflix's explanation about why it doesn't work im Mac OS X:
Watching instantly on your computer
Our apologies instant watching is currently not supported for Macintosh.
Our goal is for Netflix members to enjoy movies and TV shows on whatever screen they want. We're required to use Digital Rights Management to protect movies watched instantly online, and right now we only have approval for this protection on Windows Operating systems, not the Mac.
Apple does not license their DRM solution to third parties, which has made this more difficult, but we are working with the studios and content owners to gain approval for other solutions. As soon as a studio-approved DRM for the Mac is available to us, whether from Apple or another source, we will move quickly to provide a movie viewer that enables you to watch movies from Netflix instantly on your Mac.
In the meantime, you can use your account to watch instantly on any compatible PC, and Intel-based Macintosh computers can watch movies instantly using Boot Camp, Parallels, or Fusion to run Windows. Also, your Macintosh is fully compatible with adding titles to the Instant Queue for later watching on compatible devices, as well as to have a playlist ready to go when we do enable Macintosh watching.