I'm so used to XCode click on source file, see the file. Double clicking seems more Visual Studio like. Is there a preference option?
If you open "Options" context menu for project toolwindow, you'll see there two options: Autoscroll To Source and Autoscroll From Source. You can use them to simulate one-click source view.
But usually you don't need to navigate with project clicking. Try to use tabs, Cmd+E (recent), Cmd+N (Goto Class), Cmd+Shift+N (Goto File). It's usually much faster.