@cpaine The IP in the screenshot is random and not meant to be used as is. Neither is “os_ip” and the document does say to replace these with your OpenSprinkler IP.
Basically, you need to add “/su” after the device IP.
Opening the device address on my home network just gets the OS user interface; but there is no option there to change the Javascript URL (or if there is, I haven’t found it).
Just add, /su, after the address in the browser to get to the page described.
Update: If you are using 2.0.0 than the SD card is not supported. This method is only supported on firmware 2.0.3+.
Update 2: The guide has been updated to reflect this.