Try this:

unrar --help

That gives you a quick idea of how to use unrar. Basically it's like this:

unrar /path/to/file.rar

So....

unrar /root/Desktop/MISC.rar

realize that you cannot do much of anything in /root as user (that IS the home directory of the root user afterall!) so you'll need to move the file to somewhere you can write to or become root.