This is a very simple system for creating a level unlock system for your Unity game. It will be using PlayerPrefs to save that progress and what levels have been unlocked. I am including the project source as a link via DropBox. Feel free to open it up and play around with it, use it as a base for your own game or don't use it at all!
Project Source: https://www.dropbox.com/s/4bfr6urbd1a...
** Wanted to add this, in the video I didn't demonstrate placing the GUI Textures, but the level icons sit behind the locked icons...basically move all of your level icons back in the Z transform so they sit behind the locked icons in the scene.