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