Use the TOOLS|QuarkXPress Version|Located Installed QuarkXPress menu item to have Q++ search your system for all the installed versions of QuarkXPress installed on your system.
This feature can also be launched by clicking on the part of the Q++ Status Bar which displays the QuarkXPress version currently used.

This procedure is required after you have installed a new version of QuarkXPress and want Q++ to be able to use it.