Video padding using ffmpeg
Try it this way instead:
ffmpeg -i input.avi -vf "pad=width=640:height=480:x=0:y=120:color=black" output.avi
The video filter syntax is the new way to use padding. Here the video output width and height are 640x480 and the image is placed 120 pixels from top, 0 pixels from left. To work out your exact dimensions, just apply the same principles.
See the pad video filter documentation for more details.