Not really familiar with Mint. But generally on Linux based systems it's the following:
Use either root or sudo (IE, add the word "sudo" before the commands above) to make the commands work.
You can also try Synaptic instead of apt-get.
The other thing you may wish to do is check your sources.list. This is generally located in /etc/apt/sources.list. If it's not located there, you can find it by doing a search (IE, "locate sources.list" --> may need to run "updatedb" first).