Homebrew cask option not recognized?

I am following an online resource for installing two Mac utilities

http://www.economyofeffort.com/2014/08/11/beyond-ctrl-remap-make-that-caps-lock-key-useful/

Here is the pertinent section:

Install both Karabiner and Seil. (Here again, if you use Homebrew
Cask, you can do this with

$ brew cask install karabiner and

$ brew cask install seil

But the cask option is not recognized?

06:51:35/shared $brew cask install seil
Error: Unknown command: cask

Then, just for laughs I tried the command without the cask:

06:55:01/shared $brew install seil
Error: No available formula for seil
Searching formulae...
Searching taps...
Caskroom/cask/seil

It is not clear what the last line of output even means: nothing seems to have happened on the system.