Click on the start menu, click on all programs and scroll down to "sixteen tons entertainment" or "wizard works", click on that and then emergency 4 editor.
When you are in the editor and you click a civilian you can edit traits and properties. you can select whether the person is a zombie in one of those menus.
When you load EM4 and the screen to select campaign, freeplay etc. comes up you can select multiplayer. Find the server you want to join and click join. If you are using mods then you have to have EXACTLY the same files as the people you are playing with.