React cannot find entry file in any of the roots
Sorry for answering my own question (a bit lame), but I hope it helps somebody else.
Apperantly we need run the npm command by giving an alternative path.
npm run start -- --root <directory_with_your_react_component_js_sources>
On http://facebook.github.io/react-native/docs/embedded-app.html#content is a guide on how to integrate React Native in an existing app.
That happens when React wants to use a port that is already being used by other application. Since React listens to port 8081, you can get the process running at that port typing the following in Shell:
lsof -n -i4TCP:8081
You'll get an answer like this:
COMMAND PID USER FD TYPE DEVICE SIZE/OFF NODE NAME
node 28571 username 32u IPv6 0x742e2ab233abdb9f 0t0 TCP *:sunproxyadmin (LISTEN)
Just killed that by typing in shell
kill -9 <PID>. Use the same PID that has been provided in the last answer.