Can't open mixer and other features

I am a new iMac user and I didn't know that everything kept in Desktop and Documents is automatically going to iCloud and suddenly I found that my iCloud was full. Then I found out that the actual internal storage is "Macintosh". Then I moved everything from desktop and documents to Macintosh. What happened next? I was so happy because I had solved that problem and opened Musescore. But surprisingly, I could open the files but I couldn't play them. I can't open de mixer, the synthesiser and I can't export parts. I am desperate. This hole Apple thing is driving my crazy. I need help.