This a simple example of what I think you're after. I got the basic idea from a post from VanB in this thread:
http://forum.thegamecreators.com/?m=forum_view&t=193534&b=1
sync on
sync rate 60
autocam off
set window off
`make a board (a white box)
make object box 1, 1000,10,1000
`make a marker object (a red sphere)
make object sphere 2, 20
color object 2, rgb(255,0,0)
`main loop
do
`determines the position of the mouse projected onto the board
`if the mouse is over the board
if pick object(mousex(),mousey(),1,1) = 1
sphere_x# = get pick vector x() + camera position x()
sphere_y# = get pick vector y() + camera position y()
sphere_z# = get pick vector z() + camera position z()
endif
`position the sphere under the mouse
position object 2, sphere_x#,sphere_y#,sphere_z#
`position the camera
position camera 0,1000,-1000
point camera 0,0,0
sync
loop
Hope this helps.