unRaid is currently going through some growing pains with regards to the plug-in management (as of 9/2012). The current most popular method is the .plg ("PLG") file. The PLG file is script that goes and gets other files that make the whole install happen.
Some trouble shooting tips here
1. get the PLG into your unRaid server to /boot/config/plugins. Either download the PLG to your computer and copy it to unRaid via the network, or log into telnet and run
Where the http:// is the link to the PLG in question. Wget will download that PLG to the folder you were in when you ran the command.
2. install the PLG installplg sabnzbd-0.4c-i468-bw.plg<enter>
and the script will take care of the rest.
1. delete the .plg file (/boot/config/plugins)
2. reboot server
Make GUI more pretty
There is a very handsome package of unRaid enhancements written by a user named Speeding_ant. He calls it SimpleFeatures. You can read about it here
Installing this SimpeFeatures package and getting the Influencer plug-in set is a great way to get your unRaid box working for you.