copy mouse location to cube 2.49

Ok in 2.49 im trying to get my cube position to copy that of my mouse. However, when I run the script it returns “error setting vector, 2 args, should be 3” I think this is because my cube has positioning data for X, Y, and Z but my mouse cursor has only X and Y. So they dont match up and it doesn’t know how to set it. Is there a way to make the mouse have 3 sets of position data? Or to have the cube only copy 2 sets and exclude the Z axis? remember 2.49 API please

you want to move the cube along the viewplane?

why 2.49? 2.63+ has such nice functions for such things: