Rick,
I too am sorry for your loss.
Since you are logged in with an Forum ID that has only one post I'm going to assume you used the email associated with that Forum ID to have registered the Platinum Serial Number. You'll need to get the "My Account" section of the Cakewalk website (not in this forum) to confirm that SONAR Platinum is registered there and if not you'll need to register it there. At that point you can then authorize/activate your computer copy of Platinum.
That is the short version.
If any of the assumptions I made are inaccurate you may need to adjust that flow accordingly if you can or as James suggested email support.