Font Problems With El Capitan Upgrade

I am needing to edit an Illustrator file created with OSX Snow Leopard in CS4. Now I am using OSX El Capitan and CS6. When I open the file, I get the attached notice:

I have put the original StoneSans font in the user and various other places to no avail. Please help. I am out of trouble shooting ideas.

thank you, notatechie

Gone are the old days of just drag and drop of a new font.

In order to install a new font to your system you need to open FontBook.

https://support.apple.com/en-us/HT201749