No add-ons. Non-IWAD files you add are loaded alongside the game you pick.
Cheats
Press ` to toggle. Cheats are typed into the game for you.
Choose a game to start.
Arrows/WASD move · Ctrl fire · Space use · Shift run · Alt strafe · Enter/Esc menu · click game for mouse (left fire, right use, wheel weapons) · ` cheats · Alt+Enter fullscreen