OK - press F1 and go to the index. Look up the following two commands:
Position Object
Get Ground Height
The first shows you how to place an object (DB primitive or loaded .X file) anywhere in 3D space using X,Y,Z co-ordinates.
The second will tell you the height of the matrix at any given X,Z co-ordinate so you can use the value to place your object at the correct Y height so it's not below the matrix.
That's really all there is to it!
You know the width and height in pixels of your matrix, so any values between 0 and those values for the X and Z will place the object somewhere on the matrix.
TDK_Man