So on my CentOS 6 box I get offered php-mcrypt from either REMI or EPEL (Remi has the newer version):
php-mcrypt x86_64 5.3.3-3.el6 epel 19 k
php-mcrypt x86_64 5.4.29-1.el6.remi remi 51 k
So I would say the first things to check are to make sure you have the right version of the repos installed and make sure they are enabled:
Check your CentOS version by doing:
You should see something like:
CentOS release 6.5 (Final)
If it says CentOS 6 you need the el6 repos, CentOS 5 would be el5.
In the file you should see entries similar to this:
name=Les RPM de remi pour Enterprise Linux 6 - $basearch
Ensure that you have the right version (in other words make sure the right number is in the mirrorlist line, if it is wrong you will need to remove and add the correct repo) and that the value next to enabled is set to 1. Check the EPEL file as well (it will be in the same folder as REMI, just called epel.repo). Then save the file and try running the yum install again and see if it finds the packages.