unrar is for extracting rar files. In order to manipulate rar files (like look in them with ark), you need to install the rar package.
What distro are you running? if you're using an rpm distro:
http://rpm.pbone.net/
type in 'rar' (no quotes) into the search box.
As far as I can tell, you don't need to match versions of rar and unrar.