Simon,
I understand your frustration. I believe you need to point the software center to the key.
-1:open the terminal window
-2: enter the following command "wget
http://doc.hausser.ch/debpackages/dha-debpackages.key -O - | sudo apt-key add -"
-3:hit enter and the system will request your sudo (administrator password). Type your password.
Then go into the software center to check if the keys were installed
-1:open the software center
-2:open tthe edition menu that is located in the upper left corner of the screen (not in the software center woindow itself!)
-3:select software source or something
-4: the software source windows opens, select the authentification or key tab (I am under a french version and therefore the names might change a bit) tab were all the keys are listed. check that Dominique hauser is in the list.
Done. Then retry the installation.
Hope it helps. There is a lot of help available for linux on the net (you can start at
http://www.ubuntu.com).
Have fun.