Hi and welcome to the forum

What do you mean by 'won't open'? Mixing down a song produces an audio file like a .wav, .mp3 or several other formats (you can even have S1 produce files in several formats at once). You can play these files in media players that accept the mixdown format(s) you chose. There is no 'opening' these exports to see individual tracks etc. They are just for sharing/listening to S1's main mix outside S1.