I presume you have downloaded and installed the minimalist ISO.
If you plan to run it on any x11/xfce4/Gnome/KDE environment, I suggest you just to download the full ISO DVD and install it.
If you are on a i386 or AMD64, just select it from here:
http://wiki.centos.org/Download
So, just go
http://mirror.nsc.liu.se/CentOS/6.2/isos/x86_64/
and select CentOS-6.2-x86_64-bin-DVD1to2.torrent
After downloading via torrent your DVD iso, just install it.
Installing the full CentOS ISO DVD will install you a full desktop gnome running beautifully out of box without any headches.