Install the dependencies for the archiso package:
(root): pacman -S make squashfs-tools libisoburn dosfstools patch lynx devtools git
I recommend archiso getting them from git, there is a package in the repositories, however, at this time of writing, it will not work with the instructions below.
So, grab the most recent version from git and install it:
(user): git clone git://projects.archlinux.org/archiso.git && cd archiso