If installing using the full install: remove your existing Zombie Master directory (if any). Follow installer instructions. Install into your \Steam\steamapps\SourceMods directory, which should be detected automatically by the installer.
If installing a patch: just follow installer instructions. Make sure you have installed the appropriate full version before installing a patch.
Note about ZM servers:
A popular server-side plugin adds "upgrades" for players, an RPG-like experience system that significantly changes the original gameplay. Whichever style of play you prefer, note that Zombie Master was not intended to be played with such a system, and has not been designed and balanced to accommodate it.
Zombie Master Beta 1.2.1 full installer for Windows:
Zombie Master Beta Linux Dedicated Server 1.2.1 full:
Though the ZM team has released a public beta, this does not mean the mod is in a completely finished state. Don't expect a fully polished game.