make sure save demo is on when you go to a new race (click on new race tab in game )
Once you do a race it will save, you need to alt-tab out and go to the personal folder >demos> then copy&paste "default" and rename it to something new so it saves.
Retab back in and now THAT race you JUST did is saved, redo as much as you want.
Now you can go back to playback race and select the files you saved. This also works for demos you have downloaded on here from people, just import them into the DEMOS folder inside the personal folder.
personal folder is located in
C:\Users\***********\AppData\Local\MX Simulator
Privateer Life # 323 Support from Motosport | Race Tech | 180 Decals