If you have the diamond demo player in your demo folder, do the following:
1. Double click on the Diamond Demo player app. Soldat doesn't need to be running.
2. After it opens, double click on which ever demo you want to watch in it's little playlist. All the demos, in the folder should show up in the playlist.
3. Enjoy. DDplayer will automatically open Soldat and start the demo playback correctly.. just in case you make a mistake trying to do it manually. Yeah, this is the lazy way to do it, but it works.
Also. Welcome to the Soldat community.