atridesign yup. I'm running Sketch 64 and I have it working fine. This is what I did:
I went to https://brew.sh/ and copied the said code
/bin/bash -c "$(curl -fsSL https://raw.githubusercontent.com/Homebrew/install/master/install.sh)"
into my Terminal.
it installed Homebrew.
Then, in Terminal again, I type
brew install ffmpeg
once that runs restart your Sketch. It should work (again, I'm in v64).