CentOS に DAG(rpmforge)リポジトリを追加する方法は以下の通り。

CentOS 6.x (x86_64)

# rpm -Uvh http://apt.sw.be/redhat/el6/en/x86_64/rpmforge/RPMS/rpmforge-release-0.5.2-2.el6.rf.x86_64.rpm

CentOS 6.x (i386)

# rpm -Uvh http://apt.sw.be/redhat/el6/en/i386/rpmforge/RPMS/rpmforge-release-0.5.2-2.el6.rf.i686.rpm

CentOS 5.x (x86_64)

# rpm -Uvh http://apt.sw.be/redhat/el5/en/x86_64/rpmforge/RPMS/rpmforge-release-0.5.2-2.el5.rf.x86_64.rpm

CentOS 5.x (i386)

# rpm -Uvh http://apt.sw.be/redhat/el5/en/i386/rpmforge/RPMS/rpmforge-release-0.5.2-2.el5.rf.i386.rpm

環境:
CentOS 5.7
CentOS 6.1