Sometimes when opening a project in Xcode4, you'll end up with a blank screen "No Editor" floating in the middle of the screen:
The solution here is to look on the far right side of the toolbar and click on the "show navigator" icon (its the leftmost icon in the group of three View icons):
That will reveal the navigator on the left side of the screen. Now you should see your project listed there. Click on the arrow next to the project name to reveal your files.