It could be that the "rosetta" layer that existed in previous versions of OS X is no longer present. At least, that’s what I gather based on the confused guesses on other forums discussing the same problem with other applications. I think Rosetta was more for PowerPC support, and AFAIK 12blocks never had that anyway.
Well, if you can figure it out I’d love to know! In the mean time I’ll keep using my old PC for 12blocks development. Hmmm, now what should I make?