On a debian based distro you would simply type:
sudo apt-get install tree
But for Red-Hat you will use an RPM
Go to: http://rpm.pbone.net
and search for the package that you want to install (if it's not on your installation media).
Once you have found the rpm file download it to your machine and install as root from th command line:
rpm -i tree-whatever.rpm
Edit:- Try repo's suggestion first.