At least in the Windows version, you can adjust the elo when you're playing the engine in the Shredder GUI, but not I think when you're analyzing or running engine tournaments.
But if you can find another GUI that will run in your OS and accept UCI engines, I think you should be able to adjust the Shredder engine's elo and have it stick for all engine modes in the other GUI.
Well, looks like Shredder's ELO can't be configured and Stockfish does not work. Wow. That is lame.