OpenAFS is not part of the Arch core distribution, so you will want to fetch it from the AUR http://aur.archlinux.org/packages.php?ID=3359. Get the Tarball.
Now we'll unpack the package, and tell it to download and compile OpenAFS.
$ tar zxvf openafs.tar.gz $ cd openafs $ makepkg -s
After a considerable amount of time you will have a newly built openafs-*.tar.xz in the current directory.