Umm... you don't get them in those modes. Only way I knew of is to use the bench with your game un-patched but hell if that works still.
You don't need them by the way. Did the DLC last night (all 250:gsicon) and I didn't have horns on PS. Just used a Marauder rifle with all +2 circuits. Cakewalk.
I have moved to PC gaming, so I
won't be on Xbox LIVE anymore.