Add this to Your Info.plist file:
filename is Your .icns file inside
Contents/Resources directory. Info form the Apple Documentation
CFBundleIconFile (String - iOS, OS X) identifies the file containing the icon for the bundle. The filename you specify does not need to include the extension, although it may. The system looks for the icon file in the main resources directory of the bundle.
If your Mac app uses a custom icon, you must specify this property. If you do not specify this property, the system (and other apps) display your bundle with a default icon.