Hi @[deleted],
In order to enter your registration code please Open the program -> From the top left corner, click the "File" menu -> From the dropdown menu select "Register software..."
I have uploaded the photo on files.fm that could help you: https://files.fm/u/ycergeg5b
The registration code for the program should be on your email address. If you can not find it you can contact NCH technical support since it is confidential information.
Here is the link: https://www.nch.com.au/support/index.html