Do you have a list of repositories defined in /etc/apt/sources.list?
If so issue apt-get update and it will update the list of available packages. If not then you will need to set up some sources to download from first

That *should* have been done in the install, but it looks like you may have exited a bit early.
Go ahead and "apt-get install x-window-system", that should get you X.
If you still have problems and used the CD you can use dpkg to install the packages off of the CD.