I am trying to sort this out too on my 525 (see new question
just asked!!) So when you go into set up for activity
and select change behaviour of buttons how do you then
move them around so that your favs are first. Do you have
to overtype the first one say with what you want it to
be and then select the function from the drop down box?

If so then i presume the one you've overtyped ypu have
to add as anew button?