I\'m working on an app in Mac OS X Mountain Lion that needs to access the private key of some of my certificates within the Keychain Access. Since this app is going to run on a
The only way I found to solve this is by setting up the Private Key in the Keychain Access to "Allow all applications to access this item".

You can configure this from the Terminal when you import the certificate into your Keychain.
Import command: import inputfile [-k keychain] [-t type] [-f format] [-w] [-P passphrase] [options...]
To set the access control to "Allow all applications to access this item", just add the flag "-A".
You can check always Apple's manual on security
Hope this helps.