apt-get install fails with error "Unable to locate package"

Looks like you just haven't updated your package lists, this is missing from the link that you gave -

sudo apt-get update

This should download the list files from the repos in /etc/apt/sources.list so that apt-get install knows what packages to look for.

Note also that you should do this regularly as the repository will change over time. In particularly do it before installing software if it hasn't been done for a while!