NZBGet unpack not working

You can just download a copy of Unrar and problem solved.  I've done it before on my ReadyNAS 516.  Unrar was in the same drawer as NZBGet.  So my guess is your settings in NZBGet are not right.

So these I would look at in the PATH settings.  Main Dir, is that right?  That should be where NZBGet is installed.  I had it installed in /apps/nzbget/

So if you don't cange the other paths MainDir, they should be looking there. There's also a 7za Or ZIP file extractor also included.  If you look in the Unpack settings for NZBGet there is a UnrarCmd setting.  Mine shows ${AppDir}/unrar.  Again if you have it going someplace else, it's not going to find UNRAR.  Then there's SevenZipCmd which mine shows, ${AppDir}/7za

Again {AppDir} in the Directory NZBGet is installed into, Unrar and 7za is located here.  There's also the scripts & webui directory in here.  So those need to be pointed in the correct location also.  A great programto look around on your NAS (File Manager) is called WinSCP.  Maybe that will be a help to you.  

To me it just sounds like you have a setting or 2 screwed up and that's why Unrar is not working for you.  It's a easy thing to do and if you're not used to Linux, same here, long time Windows user since Windows 95.  Most of my Linux learning is on my ReadyNAS 516 and a little at work, though not so much these days.