[nmglug] Help, please, with installing from repository

Hi NMGLUG folks, I could use some help with installing programs from the repository. Ideally, I'd like to learn how to do this from the CLI. It would be good to be able to do it also from the GUI "Software". In this GUI, I found, for example, Kdenlive and Openshot. I looked for Avidemux, but did not find it. For Kdenlive and Openshot, when I clicked on "Install", I got a box saying that debian-oldstable-main needed to be enabled. I clicked OK. I was prompted for my password. I entered it. Then I got a box saying E: Could not find the repository. Any ideas? LeRoy -- I am the Love of God, no matter what. LeRoy Diener 213-LEROYIZ 213-537-6949 www.leroydiener.com/