this is not a big deal. As you seem to be no programmer, may I suggest that you start coding using the C language - this is much easier than changing the assembler code for MIDIbox 64.
Take a look at the LED controlling example and see if this makes sense to you. If so, you should go to the WIKI to install the toolchain and start coding. It is really simple and straight forward, and what you need is easily implemented even without programming experience by just following the implementation example above.