Please follow the below steps in the same sequence as mentioned below.
1. Open the downloaded qxp2015_mac.dmg
2. You will get the below Screen. DO NOT
double click on QuarkXPress 2015 icon in the below dialog.
Drag the QuarkXPress icon onto the Applications folder.
3. Once QuarkXPress 2015 has been moved into the Applications folder, launch it to execute Setup Assistant.
5. Once you launch QuarkXPress from the above screen you will get following screen. Click Continue
6. Select "Agree" for the license Agreement
7. Select "Full Install" and enter the validation code
8. The next screen will ask you to enter your details to personalize your copy of QuarkXPress, Click continue and following screen will appear
9. Click continue and the Setup will require your Username and Password for the Mac OS.
10. Click Ok and choose Continue to complete the setup