Download Quicktime from Apple. Make sure it is the same version of QT that you have before. If there is an uppgrade then you would need to buy a new Prokey. For example the key for QT6 willl not work with QT7.
Install and lunch QT - there is a registration option where you can input your QT Pro key.
I just had this problem too and so far have found no helpful responses. After about 1/2 hour of fiddling around on the computer, I finally found how to enter my current key into the free version to unlock the Pro features.
Yes you do need the free version already installed but no-one seems to tell you where to go to enter your key once it's installed. Once you know where it is, it's really simple (but I have to wonder if it's made deliberately ambiguous so that people will give up looking and just purchase a new key), so here it is:
If you don't already have the latest version of the basic free Quicktime on your computer, download it here:
Once you have downloaded and installed it, open Quicktime.
Go to Edit --> Preferences --> Register.
The Quicktime Preferences screen will pop up. Enter the name of the person the product key is registered to under "Registered to" then copy and paste your original product key into the "Registration Code" box.
Hit "Apply", hit "OK". You're done!
Now when you go into the Quicktime menus all the Pro features will be enabled
Hope this saves others with this problem some time.