how to install Mysql?

This is not related to AEM but like before you need to use yum to make life easier.

yum install <packagename>
(where packagename = php-mysql-5.3.10-2.el4.remi.x86_64.rpm i guess or normally just mysql_server / mysql )

If you have not registered your RHEL to RHN you will have difficulties with yum unless you add the installation DVD as a repository as well.
For more linux related information I would suggest you head over to the linux forums.
Hope it works
/Johan