As Archlinux uses a 'rolling release' model, it is usually best to build new packages on your system. The /home/user/build directory is a good location to extract the latest src.tar.gz. You can get the latest release or svn version here: http://www.isentrix.com/pub/zoneminder/archlinux/src/ Running "makepkg" in the build/zoneminder* directory will create a new package built against all the currently installed shared libs.
The latest precompiled zoneminder package can always be found at http://www.isentrix.com/pub/zoneminder/archlinux/i686/
The latest release packages (1.24.x etc) will apply against the latest stable ZMLarch. The latest svn packages are for the latest svn iso.
After downloading the package, install with the command "pacman -U <package_file>" Then issue the command "rczm restart". If the update is a new zoneminder version you will see a message regarding updates to the database. Watch for any error messages.
If you want to follow the development version of zoneminder there is a simple proccess to keep up with the svn version. Begin by downloading the latest zoneminder-svn-xxxx.src.tar.gz from http://www.isentrix.com/pub/zoneminder/archlinux/src/ Extract it (/home/user/build is a good location) an cd into the zoneminder-svn directory. The command "makepkg" will fetch the latest svn version, compile it, and create a package file in the current directory. Remove the stable version of zoneminder with the command "pacman -R zoneminder" Install the svn version with "pacman -U <package_file>" Subsequent updates can be built by simply rerunning makepkg in the zoneminder-svn directory and installing the resulting package. Changes to the database structure during a development cycle may require reinitializing the database with "sudo /usr/lib/zm/bin/zminit". This will of course result in loss of your configuration and saved events.
All of the above can be done from the liveCD if you have enough RAM (512MB should be enough). Use this feature if you can to test and provide feedback for the latest development revision of zoneminder.