React cannot find entry file in any of the roots

Check for the node server running in one of the bash terminal, this was probably kicked by previous ReactNative XCode project you launched earlier. Stop that process and run XCode project again, this should fix the problem.

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 shellkill -9 <PID>. Use the same PID that has been provided in the last answer.

Then run npm start.