Most of the time you can leave these settings blank, and they will bind to all interfaces.
Proxy Hostname: This is your primary Machine IP and the one which is currently being used by your MTA on port 25
MTA Hostname: Localhost (Make sure your MTA is bound to localhost on port 25 before MWES is restarted).
Web Hostname: Where you will access the web gui. Can leave this blank. If you add your domain here, make sure it is bound to the machines IP address, otherwise you will lose access to the GUI, and have to consult the troubleshooting section...