Open Terminal in OSX Enter the command: cd music/itunes/"mobile applications" Enter the command: ls You should see Doom.ipa listed. If not then you need to make sure that Doom is backed up to your iTunes Library Enter the command: cp Doom.ipa Doom.zip This copies and renames your iTunes backup of Doom Open Finder Go to the folder: /Users/username/Music/iTunes/Mobile Applications/ (where "username" is your username) You should see Doom.zip listed Double click Doom.zip to uncompress Navigate to the new folder (should be named "Payload") Right click on Doom.app and choose "Show Package Contents" This will open a new Finder displaying the contents of Doom.app Copy the base folder Paste the base folder in the DoomSource folder Open the Doom xcode project located at ../DoomSource/code/iphone/Doom.xcodeproj Click on "Build and Go" or press "command + R" to build and then run Doom