1. download the two files (Add-NASM.sh, NASM.xclangspec)
2. Open Add-NASM.sh and change the DVTFOUNDATION_PATH if necessary. You probably do not need to change this if you are on Xcode 7.2.1+
3. Make sure Xcode is closed
4. Run Add-NASM.sh from the terminal with sudo (I.E. 'sudo Add-NASM.sh')
5. Enter your admin password and hit enter
6. Open Xcode and notice the 'NASM' entry under "Editor->Syntax Coloring"

Special thanks to araxara for the Objective-J version of this code.