To register your license, open the program, at the top left corner of the program, from the drop-down menus, go to "File > Register Software...", enter your registration code (numbers-letters) and click "Register".
If successful, you'll see the "Licensed software" status at the very top of the window, next to the program's name, sometimes a program restart is needed to reflect this change.
If you receive a "Too many installs" error message when registering your license, please enter the numeric portion of your registration code at http://www.nch.com.au/kb/10078.html and try again.
If you still getting issues, contact the Technical Support here: https://www.nch.com.au/support/supportcontact.html?software=Doxillion