Check the IP address on the right corner of the printer, shown in the below picture.

Open Wiibuiler click the 'octoprint icon' and type into the IP address, and click 'connect'

Please make sure your computer connect into the same internet with the printer.

You can slice the stl file to Gcode file and click 'Upload And Print' then the Gcode file will upload to the printer and start printing after upload finished.