If you are using the yum install for xCAT 2.7 on CentOS 6, you will need the .repo and putting in /etc/yum.repos.d/

# wget http://sourceforge.net/projects/xcat/files/yum/stable/xcat-core/xCAT-core.repo
# wget http://sourceforge.net/projects/xcat/files/yum/xcat-dep/rh6/x86_64/xCAT-dep.repo

Do a yum check-update

# yum check-update

Do a yum install of xCAT ie

# yum install xCAT

You might get the error

Error: Package: xCAT-2.7.2-snap201205230215.x86_64 (xcat-2-core)
           Requires: elilo-xcat
Error: Package: xCAT-2.7.2-snap201205230215.x86_64 (xcat-2-core)
           Requires: xCAT-genesis-x86_64

You will notice you will have these error. To rectify, you have to download the from http://sourceforge.net/projects/xcat/files/yum/xcat-dep/rh6/x86_64/ and do a rpm install

# rpm -Uvh xCAT-genesis-x86_64-2.7.......
# rpm -Uvh elilo-xcat-3.14-4.noarch.rpm

Finally do a yum install xCAT and you should be able to install without issue.