Not a 3dsmax user but generally speaking...
if your using the top right photo as your alpha when compositing then the entire clip is acting as an alpha inlcuding your shadow and the ground plane which is probably not what you want.
I would have a seperate alpha for the object and render the shadow in a seperate pass and composite them with different blending options.
Google around for "3dsmax multipass rendering".
Thanks for reply Gord...well, im using my top right photo as alpha, it was render separate from the original picture, and for the plane object i use -1.0 matte object but still im getting this grey shadow instead a black shadow.
I will look forward for Multipass rendering as you said....thanks again!.