The best route to go is not to use MouseX(), MouseY(), but to move the box along the x and z axis using MouseMoveX() and MouseMoveY() respectively. Like this:
Do
xMove#=MouseMoveX()
zMove#=-MouseMoveY()
Position Object 101,Object Position X(101)+xMove#,Object Position Y(101),Object Positon Z(101)+zMove#
h#=Get Ground Height 1,Object Position X(101),Object Position Z(101)
Position Object 101,Object Position X(101),h#+Object Size Y(101)/2,Object Position Z(101)
`...
Loop
"Droids don't rip your arms off when they lose." -H. Solo
REALITY II