You need to install yum-utils to use yum-config-manager

[root@gitlab ~]# yum-config-manager --enable rhel-6-server-optional-rpms
-bash: yum-config-manager: command not found


[root@gitlab ~]# yum search yum-config-manager
Loaded plugins: fastestmirror
Loading mirror speeds from cached hostfile
 * PUIAS_6_computational: www.puias.princeton.edu
 * base: mirror.skylink-datacenter.de
 * epel: mirrors.n-ix.net
 * extras: mirror.skylink-datacenter.de
 * passenger: mirror.hmdc.harvard.edu
 * updates: centos.mirror.linuxwerk.com
epel/pkgtags                                                                         | 902 kB     00:00
======================================= Matched: yum-config-manager ========================================
yum-utils.noarch : Utilities based around the yum package manager