This is a simple copy of NESAsm 3.1 with an added makefile for OS X. You will need to have Xcode installed (the Makefile uses clang to compile the files).
I do not own any of the code, none of it has been modified. The compilation throws some warnings, but still succeeds. The license header is copied below.
original license header
MagicKit assembler ---- This program was originaly a 6502 assembler written by J. H. Van Ornum, it has been modified and enhanced to support the PC Engine and NES consoles. This program is freeware. You are free to distribute, use and modifiy it as you wish. Enjoy! ---- Original 6502 version by: J. H. Van Ornum PC-Engine version by: David Michel Dave Shadoff NES version by: Charles Doty