Ah, you know what? I totally missed the 3D part. Just skimmed right over it. Forget I said anything.
The short story: The 3D Box doesn't have collision detection, at least not the kind you're probably thinking of. Don't use the 3D box as a game object. It's for display only.
The long story: Okay, fine. You can use it as a game object, as long as you're clever about it. But I don't feel like typing the long story out (again). I've written up the 3D Box description a zillion times already.
Oh what the hell, I'll dig up a link for you...
Okay, here you go:
Anyway, if you want to do collision stuff with the 3D box, use a regular, invisible sprite as your collider. Perform all your game actions on the sprite instead of the box. Then just put in a "Always: 3DBox: Set position to object colliderSprite (image point 0)." Voil�. It looks just like you're manipulating a 3D Box directly, and nobody's the wiser.