'Module not found' when trying to load any module

Ahh, right - I think this is because you're typing code in the left-hand side and the Web IDE doesn't automatically look at what you type there and load the libraries - each character goes right to Espruino, so by the time you've hit enter on a command, it's too late to load the library.

Your best bet is to simply type require("DS18B20") on the right-hand side and click thesend to espruino button. Then you can type on the left as you want.

I'll update the DS18B20 docs to mention this...