Hey, I'm sorry to hear that. I'm not sure about the exact reason of problem, but try to do it by instruction:
1. Press "Download" zip for ur computer os
2. Unpack zip with help of any comfortable program for u (for example, 7zip or WinRAR) in the folder.
3. Open the folder and find "DEMO.exe"
4. Open demo.exe and done!
p.s. not sure about mac os, but should be similar ig
Hope it'll help!<3