Don't put the dmg file in your applications folder. put the dmg on your desktop, then open the dmg and move the planeshift file inside of the dmg file into your applications folder. The dmg file is like a read only had drive so you have to move the planeshift folder off the dmg, which you can't write information to, and put the planeshift folder into your applications folder. Once you do that, and run the game from the applications folder, then you can write to the files and will be able to update and will not get the admin message.