Lost in FFMPEG (downloading & installing)
The easiest way to convert video on the Mac is to use Handbrake. It is easily downloaded and installed and includes the same libraries as ffmpeg. (I know you say it doesn't work in your case but I include it here for others who might end up at this question.)

The version of ffmpeg you want is the 64-bit static build, preferably in a DMG as that is the easiest one to install. Using homebrew would be an even easier way to get it installed.

As for homebrew it is fairly well explained on their home page. You don't actually need XCode, just the XCode command line tools which homebrew will download for you the first time you use it to install something.

Once you install ffmpeg then man ffmpeg will get you the manual for it. You might prefer 

man -t ffmpeg | open -f -a /Applications/Preview.app/

which will open the man page in Preview where you can read it more easily, or even print it.