The first thing to check is to verify the IP address is correct: click button B1 on OpenSprinkler to display the IP address (And port number, in case the port number is not 80). Make sure the IP address matches what you typed into the browser; also, if the port number is not 80, you need to explicitly type the port number (separated from the IP address by a colon).
Another possible issue is to check whether your network has Internet connection. Because the Javascripts required to render the homepage are stored on cloud server, whatever browser you are using to access the controller should have Internet access. Otherwise, try the OpenSprinkler mobile app, which embeds the Javascripts and hence does not need Internet access.