Why have I the message:
Failed to open descriptor file '/Users/Shared/UnrealEngine/4.8/ElementalDemo/ElementalDemo.uproject'
when I try to open ElementalDemo?
My env is: Mac OS X 10.10.2.
Well I removed the project a few times and recreate it. And it finally opens and starts. But when I run it I can move my character. It seems that keyboard and mouse don’t active in the demo. I’ve checked Project Settings - there are keyboard (wasd and etc) settings. Don’t know what’s the problem. Other projects work fine.
They surely disabled the input in one of the blueprint (level blueprint), because it’s a cinematic. To check it, just go to file - new level- choose a template level - try if the mouse and keyboard are still not active. 
It could also be that you just have to change the gamemode -> world settings - gamemode
Can you tell me exactly what you mean by “removing the project.”
Did you delete your project in the editor or do you mean you constantly re-packaged it?
I’m having the same problem and it’s extremely upsetting.
~ Jason