Here are the steps that I have taken in the past to fix the “public key is not available:” message while trying to use apt-get:
1.) Use apt-get to install debian-archive-keyring
apt-get install debian-archive-keyring
2.) Try using apt-get again. Additional information will be provided.
3.) Use additional information from last attempt with gpg commands.
Replace the KEYNUMBER with the actual keynumber that you should now have from running apt-get again.
gpg --keyserver wwwkeys.eu.pgp.net --recv-keys KEYNUMBER gpg --armor --export KEYNUMBER | sudo apt-key add -
4.) Update apt-get to resynchronize the package index files.
5.) You should be good to go!
Keep in mind you may need to run apt-get update a couple of times. Also, make sure your date is set correctly.