Add C:\OpenCV\3rdparty\ffmpeg\ to the Windows PATH environment variable or copy opencv_ffmpeg.dll from that directory to C:\Python27\ or to a directory that is in the PATH. Alternatively, use the OpenCV binaries from http://www.lfd.uci.edu/~gohlke/pythonlibs/#opencv.
Renaming the opencv_ffmpeg.dll file may also be necessary.
For OpenCV version X.Y.Zopencv_ffmpeg.dll ==>opencv_ffmpegXYZ.dll
For 64-bit OpenCV version X.Y.Zopencv_ffmpeg.dll ==>opencv_ffmpegXYZ_64.dll
