Yes, I've run into this myself. Have you found anything yet? I'm waiting for the HIARCS GUI to be released sometime this year, but it doesn't help in the short term!
Sigma Chess
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I have found that the software Stockfish can use other UCI chess engines wiii
Sigma=Seal
That might be frustrating. Well, I don't have a lot of info about this. But a little research gave me these names that you can try if you haven't already: Scid vs. PC, XBoard / WinBoard, Shredder Chess, and ChessX.
OMG!!! Sigma Chess doesn´t work with OSX Lion, does anyone can recommend a nice GUI for UCI engines on this system?