ffmpeg -i test.mp4 -vf "pad=width=300:height=170:x=0:y=40:color=black"  testcrop.mp4

 ffmpeg version 0.8.15-4:0.8.15-0ubuntu0.12.04.1, Copyright (c) 2000-2014 the Libav developers
  built on Aug 10 2014 18:16:51 with gcc 4.6.3
*** THIS PROGRAM IS DEPRECATED ***
This program is only provided for compatibility and will be removed in a future release. Please use avconv instead.
Input #0, mov,mp4,m4a,3gp,3g2,mj2, from 'test.mp4':
  Metadata:
    major_brand     : isom
    minor_version   : 512
    compatible_brands: isomiso2avc1mp41
    creation_time   : 2013-04-25 09:56:24
    encoder         : Lavf53.21.1
  Duration: 00:00:32.38, start: 0.000000, bitrate: 346 kb/s
    Stream #0.0(und): Video: h264 (Main), yuv420p, 300x170, 141 kb/s, 25 fps, 25 tbr, 25 tbn, 50 tbc
    Metadata:
      creation_time   : 2013-04-25 09:56:24
    Stream #0.1(und): Audio: aac, 48000 Hz, stereo, s16, 200 kb/s
    Metadata:
      creation_time   : 2013-04-25 09:56:24
File 'testcrop.mp4' already exists. Overwrite ? [y/N] y
[buffer @ 0x13fe840] w:300 h:170 pixfmt:yuv420p
[pad @ 0x1406160] Cannot find color 'color=black'
Error initializing filter 'pad' with args 'width=300:height=170:x=0:y=40:color=black'
Error opening filters!