I had the same issue, and found the previous suggestions gave me enough info to get it working on my server. The following is what I did:
1) Install EPEL repository (From: http://www.rackspace.com/knowledge_center/article/installing-rhel-epel-repo-on-centos-5x-or-6x):
wget http://dl.fedoraproject.org/pub/epel/6/x86_64/epel-release-6-8.noarch.rpm
wget http://rpms.famillecollet.com/enterprise/remi-release-6.rpm
sudo rpm -Uvh remi-release-6*.rpm epel-release-6*.rpm
2) Install it via:
yum update
yum install php-mcrypt*
The yum update updated the repository reference on my server then the 2nd command got what I needed 🙂
3) Restart your webserver. For my apache install I did:
service httpd restart