I had trouble with the new owncloud client (1.4.0) too.
You can try once agein to use the command apt-get update and then apt-get install owncloud-client
I am afraid, that this doesn't work.
Then you can try to download all *.deb files from the repository (if you use 32-bit Ubuntu):http://download.opensuse.org/repositori ... 2.04/i386/
After that you use the Ubuntu software manager to install the *.deb (right mouse click on the file).
You should start with the lib*.deb
the second to last should be the ocsync_0.90.0-1_i386.deb and the last one the owncloud-client_1.4.0-1_i386.deb
After that you should start the owncloud client in the GUI.