Getting the MLton compiler to install on OS X was pretty straight forward but I've never been able to get Basis to load properly.
I also recently burned an afternoon trying to install MLKit on OS X. This one seemed promising for its good docs, smltojs, and good FFI so I was bummed I couldn't get it work.
SMLNJ installs pretty easily but holy cow do you have to murder your PATH.
Eventually I settled for MoscowML which is a VM rather than native code but it has provided me the best ML on OS X experience so far. I also enjoy having the REPL.
Yours truly,
ML compiler/VM collector