I posted this on my blog a few months back, but I am re-posting it here as well.
This will only work on Debian based systems.
Firstly you need the good old terminal window open. Once that has opened type:
sudo apt-get install rar
You should now be asked for your admin password. If you are not the admin then you will need to ask them to install it. And that's it, RAR archiving will now work seamlessly with Archive Manager!
Hope this helps,
Edited by James Brooks, 05 July 2009 - 13:57.