Download and put it in EditPlus instalation directory.
Open EditPlus and do Document – Permanent Settings. There, choose Files – Settings and Syntax. Under File types, do Add, choose the description you want (ex. Assembly) and ASM as the File extensions. All you have to do now is enter the syntax file asm.stx path under “Syntax file:”
Open an .asm file now, it should be syntax colored. ;)