It worked, thanks! I had a numeric pad connected to the Mac, I never thought that would be related to the problem, specially only with Calibre (other apps worked just fine). I still don't know why that generates a problem, but at least now I know how to stop it.
I downloaded from here:
http://calibre-ebook.com/download_osx which I understand is the official site... right?
Thank you again for your help, I'd probably never figured it out on my own.