There is an extra step you need to take now before you can upgrade, as the repository has been modified since that ISO came out.
As you have just installed, it might be quickest to re-install before going any further, so you hacve a fully functioning system.
Then open Synaptic, reload, and search for the package "apt-sources-list". Mark that for installation and apply. Once that is installed, open Settings->Repositories and select the repo you want to use. Then reload, and Mark all upgrades etc.
This extra step is needed to include a new repo section which didn't exist when the ISO was made.
Obviously we need a new ISO, but the lead developer is off sick at present which will probably delay that a bit.
In the meantime, perhaps the instructions need modifying to add the extra step.