The answer is
Here's a video and explaination how to use Instruments and NSZombie to find and fix memory crashes on iOS: http://www.markj.net/iphone-memory-debug-nszombie/
To enable Zombie logging double-click the executable in the executables group of your Xcode project. At this point click the Arguments tab and in the Variables to be set in the environment: section, make a variable called NSZombieEnabled and set its value to YES.
Product > Profile will launch Instruments and then you there should be a "Trace Template" named "Zombies". However this trace template is only available if the current build destination is the simulator - it will not be available if you have the destination set to your iOS device.
Also another thing to note is that there is no actual Zombies instrument in the instrument library. The zombies trace template actually consists of the Allocations instrument with the "Enable NSZombie detection" launch configuration set.
In Xcode 4.5.2 goto Product -> Edit Scheme -> and Under the Diagnostics tab check the check box in between Objective C and Enable Zombie Objects and Click on OK
I encountered the same problem with troubleshooting EXC_BAD_ACCESS and had hard time to find the setting with Xcode 4.2 (the latest one that comes with iOS5 SDK). Apple keeps on moving things and the settings are no longer where they used to be.
Fortunately, I've found it and it works for the device, not just Simulator. You need to open the Product menu in the Xcode, select Edit scheme and then choose the Diagnostics tab. There you have "Enable Zombie Objects". Once selected and run in debugger will point you to the double released object! Enjoy!
In short
Product->Edit Scheme->Diagnostics-> Click Enable Zombie Objects
As of Xcode 3.2.5 and Snow Leopard (Mac OS X 10.6), you can run your code through the Zombies instrument: Run > Run with Performance Tool > Zombies. That allows you to see particular objects and their retain counts on a timeline.
in ur XCODE (4.3) next the play button :) (run)
select : edit scheme
the scheme management window will open
click on the Arguments tab
you should see : 1- Arguments passed on launch 2- environment variables
inside the the (2- environment variables) place
Name: NSZombieEnabled
Value: YES
And its done....
In the preferences of your executable add the environment variable NSZombieEnabled and set the value to YES.
In xcode 4.2
Goto, Product -> edit scheme -> click Run yourappname.app -> Diagonostics -> Enable Zombie object.
Go to Product - Scheme - edit scheme - Arguments - Environment Variables set NSZombieEnabled = YES
It's a simple matter of setting an environment variable on your executable (NSZombieEnabled = YES), and then running/debugging your app as normal.If you message a zombie, your app will crash/break to debugger and NSLog a message for you.
For more information, check out this CocoaDev page: http://www.cocoadev.com/index.pl?NSZombieEnabled
Also, this process will become much easier with the release of 10.6 and the next versions of Xcode and Instruments. Just saying'. =)
In XCode 4.0: To detect NSZombie in Instruments, select the Simulator as your target (can't detect NSZomboe on device). Run Instruments (CMD+I) and select "Zombies" trace template. Enjoy.
Product > Profile will pop up Instruments. Select zombies from the panel and go nuts.
NSZombieEnabled is used for Debugging BAD_ACCESS,
enable the NSZombiesEnabled environment variable from Xcode’s schemes sheet.
Click on Product?Edit Scheme to open the sheet and set the Enable Zombie Objects check box
this video will help you to see what i'm trying to say.
Source: Stackoverflow.com