For some reason, the ArchLinux wiki is kind of outdated explaining how to install xmonad in Arch. Also, the new packages seem to have a bug and the xmonad installation won't work out of the box. Here you have detailed steps on how to install and run xmonad on ArchLinux. First of all, you need to install the Xorg server. Refer to Xorg for detailed instructions. After that, you'll need to install the xmonad packages, named xmonad and xmonad-contrib.

pacman -Syu xmonad xmonad-contrib

This will install xmonad and all the required dependencies. After that, if you want a fairly simple X setup, add the line xmonad to your ~/.xinitrc file (hopefully you'll be using a display manager that uses that file like SLiM). If you try this on a freshly installed ArchLinux system, though, it won't work. There are some missing steps to do. First of all, you need to create the folder ~/.xmonad.

mkdir ~/.xmonad

After that, you need to create a new xmonad config file, called xmonad.hs</em into that folder, so fire up your favourite editor and fill the file with this basic (empty) configuration: </p>

import XMonad

main = do
    xmonad $ defaultConfig

Once you have you configuration file in place, it's time to rebuild xmonad:

xmonad --recompile

And that's all, you should be able to start your fresh new xmonad setup in ArchLinux. By the way, if you wonder why pressing Mod1 + Shift + Enter does nothing, make sure you have the package xterm installed.