(This is unrelated to the screencast - you’re just trying to get Sage working)
Looks like Bower wasn’t installed
“Bower components directory does not exist”
Because bower install failed since Bower isn’t installed
Try installing Bower again (npm install -g bower) then running bower install before you try to run gulp.
These issues are also unrelated to Sage itself, see Npm Debugging Guidelines (Failed npm install, bower install, or gulp build? Read this)