If I remember correctly, the installation of Quark 6 only required a legitimate serial of Quark 5.
*Goes and digs out the disk*
On your upgrade sheet that came with the disk, there should be the Quark 5 (old) serial, a validation code and a new serial. You will need these 3 codes.
However, after installation you will also need to activate it by either telephoning Quark or by doing it online. Without activation, the program will cease saving, printing, collecting for output, and creating PDFs within days. Activation can only be done for 2 computers with one version of Xpress.
Important: The installer of Quark 6 will mess with the permissions of the entire directory it is installed into. Best to create a new folder on the desktop, label it Quark 6, install into there and then move the installation into Applications when it's done.