I have a problem.
I have just installed ffmpeg on my mac using home-brew. However, LRTimelapse does not recognise this, and says "Error: could not launch the external encoder". My mac is running on el-capitan. How can i Fix this?

There is no need to install ffmpeg manually for the use with LRTimelapse. If you use any other ffmpeg version then the officially provided one, you'll do it on your own risk, it might not work as expected.

Please just reinstall LRTimelapse, it will install ffmpeg to the right location and work by default. The location for the external encoder is defined in the LRTimelapse settings. The default is /usr/local/bin/ffmpeg