It should drop them to the top as is. Why is it doing that I don't know. Do you have more stuff under it like a real Vue terrain? Maybe hide that on a hidden layer. You can do it the hard way too and that is use a object for the top. Say make a cube and place it to the hight that you want the trees to plant. Then drag the cube up in the air create trees group with cube and then drop the group as one object. It should step right threw that object just like any other object. You could try to convert the shape maybe to some other format. Sometimes on complex objects top is recorded as center. There is really no top referance saved with the object. Try to convert it to dxf of obj and try again. What your trying to do works, why it's not working for you I don't know. I've taken imported objects and stacked them up on top of each other by dropping them. Are you sure you over top of the 3ds object before you do the drop? Check it on the up view. Click drop only once. You have to drag stuff up in the air before you do the drop. If it's sorta inside the 3ds object it will not be the ground for Vue and what ever is under it will become the new ground.