how to install certain packge with certain version using yum

Hi all,
I've installed yum-NetworkManager-dispatcher-1.1.16-13.el5.centos.noarch  using
yum install yum*

but I need to install yum-NetworkManager-dispatcher-1.1.16-12.el5 instead
when I try yum downgrade
I got this  message

[root@localhost ~]# yum downgrade yum-NetworkManager-dispatcher  
Loaded plugins: aliases, changelog, downloadonly, fastestmirror, filter-data, keys, kmod, list-data, priorities, protect-packages, protectbase, security,
              : tmprepo, verify, versionlock
Setting up Downgrade Process
Loading mirror speeds from cached hostfile
 * addons: centos.fastbull.org
 * base: centos.fastbull.org
 * extras: centos.fastbull.org
 * updates: centos.fastbull.org
0 packages excluded due to repository protections
Reading version lock configuration
Only Upgrade available on package: yum-NetworkManager-dispatcher-1.1.16-13.el5.centos.noarch
Nothing to do
[root@localhost ~]#

can any body help and tell me the right syntax
Iam using centos 5.4

thanks in advance
Regards
shang