Well Folks, I gave up on this one: despite the fact that my mac was running perfectly on Mac OS10.4, I had to upgrade to 10.6 for the Kobo!
The Mac is still running fine on 10.6 and I can use the latest version of Calibre which was my main problem.
Thanks for the advice, for those who suggested positive solutions, especially Dioioib.
Rojofo.