OK, so in order to define the function all I need to do is enter a function with that name "function mmf.DestroyCallback(ID)" somewhere in my script? Then that will be executed automatically everytime MMF destroys the object without the need for me to do anything else??

Yes, that is all.

Though to be clear, not immediately when an object is destroyed, but at the end of the current even loop. The main purpose of this function is to help you clean up when object destruction is beyond your control (like if the destroy if too far from frame box is checked).

Thanks, now its perfectly clear. I have the function set up which is why it worked before when I had #define in there. Its strange why that didnt throw an error message though. Everything seems to be working great now, cheers.