Step 1:
You need to enable EPEL on RHEL5 or compatible distributions like CentOS before you enable RPM Fusion for EL.
Step 2: Download and install rpm package for CentOS
# wget
RPM Fusion free for RHEL5 or compatible like CentOS
RPM Fusion nonfree for RHEL5 or compatible like CentOS
For more information for Fedora and the above information, go to RPM Fusion Configuration