Down loaded Scaler3 ok but won't install on my 11 year old Mac Pro version 10.15.7

When I double click on the folder in finder/downloads it just take me through the install process again and at no point does it ask me to insert my licence number. I have already downloaded and installed Scaler3 on my other computer (not a Mac) and it installed ok but I have a problem with the sound system on that computer. So thought I’d use Scaler3 on my Mac for now.