Use iFaith to stitch a custom IPSW instead. 2831 is an outstanding issue on RedSn0w 0.9.15 beta 3. You can try using RedSn0w 0.9.15 beta 1 instead of beta 3, but I believe the issue still stands. iFaith into a custom IPSW for use with iTunes 11.0 (not 11.1) and below will have it working. If you're looking to preserve the baseband, make sure you select that option as well with iFaith.