Installing ffmpeg with homebrew?
To answer this question, you need to link up ffmpeg, but I'd run some housecleaning too.

First remove ffmpeg from your system:

$ brew uninstall ffmpeg


Now update all your brew packages and references.

$ brew update
$ brew upgrade
$ brew cleanup


Now install ffmpeg on your system, and link it up:

$ brew install ffmpeg --force
$ brew link ffmpeg


Now you should be good to go.