By default elementary OS Loki doesn't include the apt-add-repository command or a GUI for installing .deb files. You can read why on their blog.

In the interest of promoting secure software installation methods, as of this release there is no default app for handling .deb packages and the command line tool add-apt-repository is not available by default. These tools are available to install from the default repos, but the stance going forward is that software should come from vetted, secure sources or in sandboxed package formats.

Installing repositories

You can install the apt-add-repository command with:

sudo apt install software-properties-common

It's as simple as that. Now you can use the command.

sudo apt-add-repository ppa:myrepo

Installing .deb packages

Loki also doesn't include a GUI for installing .deb files. But you can install them with the apt command in the terminal.

sudo apt install ./<myapp>.deb

Note the ./. You must include this if you specify a relative path.

If you want a GUI for handling these, you can install gdebi. This will allow you to click on .deb files and enter a graphical installer.

sudo apt install gdebi

That's all!