I had one or two minor glitches when i started. Did you do the following!

1] I had a disk ver 6.10 with a book. Haven't tried the downloaded iso
BUT i have heard that one needs to do the burn at a slower speed than
one would usually copy data to a CD.
2] after installation one needs to update the package... whichever
version you have there will have been some upgrades. You can use either
apt-get, aptitude or synaptic package manager. However, you must upgrade
EACH one of these separately.

Easiest is to use synaptic. Go to Synaptic> Settings> Repositories>
Ubuntu x.xx> check boxes for universe, main, multi-verse, restricted,
souces code (suggest leave unchecked unless you want to download source
code)> Close

3] Click reload. and exit synaptic.
4] A box appears top right of screen saying updates - click on it and
voila you have synaptic loaded with all the updates available from the
Repositories.
5] Open synaptic again and click mark all upgrades.

You can now select through synaptic anything you want to add. Assuming
you installed Ubuntu and now want to add Xubuntu or Kubuntu go to
Sections> Meta packages and select.

Hope this helps. I couldn't add Xubuntu at first but i think i had a
glitch somewhere first install. I suggested using synaptic as it's
recommended for the novice, apt-get is command line language and that
takes a while to grasp, aptitude is a poor interface but as i am now
learning has one or two advantages over the others and is my preference
now i've done a fair but of testing!