If your Cmake cannot find FFMpeg for OpenCV 2.1, try to do the following!

Modify your ".bash_profile" like the following:
First you need to make sure the FFMpeg libraries you installed can be found by your Cmake;
Then, make sure your "pkgconfig" can be found!
It works for me!

PATH=$PATH:$HOME/bin
LD_LIBRARY_PATH=/usr/local/lib:$LD_LIBRARY_PATH
export PKG_CONFIG_PATH=/usr/local/lib/pkgconfig
export PATH
export LD_LIBRARY_PATH