MLton does not yet support 64-bit architectures. Current status: http://mlton.org/pipermail/mlton/2005-December/028421.html http://mlton.org/cgi-bin/viewsvn.cgi/mlton/branches/on-20050822-x86_64-branch/
There is currently some experimental support in MLton, but I will probably hold off until it is more stable.