It's on RPMforge. See this link how to enable this in yum:
http://wiki.centos.org/AdditionalResources/Repositories/RPMForge

Then, you can install using the command you've tried above.

I assume that you are using the binaries. If you did compile it yourself, see masterOfDisaster's comments on how to disable UPNP completely.

http://pkgs.org/centos-6-rhel-6/repoforge-x86_64/miniupnpc-1.5-1.el6.rf.x86_64.rpm.html