My guess is you haven't downloaded tar.

(replying as I read this thread - two pages still to go, you may have fixed this by now!)

EDIT: I see you're using a directory called ~/sin despite what the instructions say to use... the nupgrade.sh script expects tar to be in the ~/bin directory. If the tar file is not downloaded to ~/bin, you will need to edit the nupgrade.sh script to change /home/user/bin/tar to /home/user/sin/tar in order to compensate for your modifications.