You don't need to install anything separately. Here are some steps you could take:
1 You have to make sure your device is connected to the same network as your pc
2 Make sure your app is running on port 8080
3 Check your firewall / security settings
No, you do not need to install Watchman. It should startup without any further actions. Just creating any new Native Mobile Quickstart App should be up and running without any other actions.
The only problem I have once in a while at starting a native app is when another app is already running locally, or was and did not end gracefully, and port 8083 is already occupied. You can check if that is so by running this at your command-prompt ‘netstat -ano |file “8083”’ and ‘taskkill -PID <pidnr> /F’ to kill the blocking process. But seeing your error, I don’t think 8083 is already in use.
You are using version 11.1 (8.11.1 I think). Try installing Mendix version 8.12.1 and repeat the creation of the native app. I expect this will solve your problem.