First you need to install FFmpeg. If you don't know how to do that, check out this link.
If you've installed FFmpeg, your problem is probably that the Replay Mod doesn't know where it is. The documentation says to find the path to the executable and type that in the left input of the Command Line Settings, but if you're having the same problem as I had, you probably don't know where it is. Well, there's an easy way to find it. Open up Terminal and type in "ffmpeg". If you have it installed you should see a lot of text. We aren't interested in most of this. What we are interested in is the text at the start of the configuration section right after the word prefix. It should look something like this: /usr/local/Cellar/ffmpeg/2.7.2_1. This is part of the path you need to put into the command line. However, if you copy and paste this into the command line, it still doesn't work. To make it work, you'll need to type "/bin/ffmpeg" after it.
And there you go! It should be working now. If it still isn't, then I'm not sure what to do, so sorry.