Doing collison depends, are you doing this as 2 sprites intersecting or in a 3D world when 2 models collid?
In the first case, which I think is the case you need help on:
assume 'ball' is a sprite, and that 'wall' is another sprite.
if sprite ball intersects sprite wall then
This IF statement will see if one sprite collides with another.
If you want to do this in director 3D then do the following:
assume 'obj1' and 'obj2' are references to models in the 3d world ( member(which member).model(which model).
ob1.addModifier(#collision)
ob1.collision.mode = #mesh --Can also be box or sphere
ob1.collision.enabled=true
ob1.collision.setCollisionCallBack(#collision, me)
then write a handler which will execute any code you want to happen on collision (on collision me). See macromedia's 3D help section on more info on this stuff.