TodoMVC is now part of the Cerebral Monorepo. So you have to clone, install and setup the Monorepo first:
$ git clone https://github.com/cerebral/cerebral.git
$ cd cerebral
$ npm install
$ npm run setup
Switch to the cerebral-todomvc directory in the Monorepo and start the dev-server:
$ cd packages/cerebral-todomvc
$ npm start
Go to localhost:3000