First of all, you will have had to setup your B2 for WiFi (or else connected to your router directly with an ethernet cable plugged inside the back).
The WebUI actually runs on the B2. You get access to it through a web browser. On the B2, as info scrolls up the screen, one line will say what the IP address is. In the browser on your phone, enter in that IP address. Mine is 192.168.1.68 - yours will look similar, but probably somewhat different numbers.