The private key & cert xcodebuild is looking for are generally stored in your keychain. When you're logged in as yourself you can see your keychain with the Keychain Access GUI. Use that GUI to export the cert and private key.
Then look into using the 'security' command line tool to import the private key & certificate pair that xcodebuild is looking for into the keychain for the _www user. You may have to actually create a keychain for the _www user (I've only ever done builds with a normal user that you can login as).