Hello Andy,
the Intellibox doesn't have fully mfx support. Who told this to you? The Intellibox is comanding the mfx locomotives with the normal Märklin Motorola modus. so you have to take in account the second adress, if you want to use the functions 5 to 8. The functions over 8 cannot be reached with the Intellibox.
The normal second adress in all locos is 255. It can be changed if needed.
rgds
Stephan