First off, thank you to all that have helped me in my efforts to get Sibelius and GPO to work together. After a lot of experimenting, I finally got it to work. For those that are experiencing trouble, try this:
1) Open GPO Studio
2) Open Sibelius and disable Kontakt Silver, enable GPO Player 1
3) Close Sibelius
4) Close GPO Studio
5) Open Sibelius and start/open project
6) Open GPO Studio and select instruments.
This may appear confusing as many have said that you need to open GPO Studio BEFORE opening Sibelius, and that was true for me only when setting up the playback parameters. I tried it 'backwards' and it now works.
Now I can enter that worst demo contest!