Ideally, you can install this syntax definition through [package control](http://wbond.net/sublime_packages/package_control): Preferences/Package Control/Install Package/6502asm-sublime
Or, you can download this repository and copy the contents into a folder called 6502asm-sublime in your Sublime [packages directory](http://sublimetext.info/docs/en/basic_concepts.html#the-packages-directory).