You need to be logged in as root (rather than nick) to run the sudo adduser "username" sudo command. You need to be root to modify the sudoers file. The idea is that if your account is in the sudoers file then using sudo with your normal password will run things as root for you.
I suspect this is your problem with the fritzing install. The install script is not finding something it wants (which probably means there is a package missing) I expect there should be a path name in the mkdir command that was found in something earlier (possibly the kde-config file that it can't find) and that's why it dies. Unfortunatly since I'm not currently running on Linux I can't easily look to see what it wants.