Since NZB files are plain text, you can “open” them with any word processor. However, you are probably wanting to do more than just look at the NZB. You probably want to download the files that the NZB points to, right?
That is easily done. Both SuperNZB and ezNZB run on OS X, and will download files from your ISP’s Usenet “newsgroup” server once you plug an NZB file into them. If your ISP doesn’t provide a Usenet server, you can sign-up with Easynews.
Download SuperNZB if you are a power user. Download ezNZB if you are a newbie and need something that will take you through things step-by-step.