If you're on a search for usenet nzb file downloaders for OSX I would really recommend you to take a look at the special OSX Version of Sabnzbd over at br41n.com

It's packaged in an .app file and simply works pretty fine :)Simply download it, start it, go over to http://www.localhost.com:8080/sabnzbd/ and start configuration.

It does the whole unrar, par2repair stuff and seems to be in active development