Thanks, rotund ! I finally sorted this out : this was a typo in the serial/activation code i received. Everything is ok now and it wasn't steinberg's culprit. By the way, i did not received any dowload links from steinberg, but my trial, already installed on my computer, was converted in a full licence and everything works fine. So i suppose you just have to download the trial, install, buy the licence and it's ok.
W7 64, intel q6600 quad core, 8 g ram, ssd, cubase 7