Please read the Tips and notes that are available on the website, just scroll under the lessons. You can also find information on how to add a Greek keyboard here and here. Basically, you need to activate the Greek keyboard by accessing your computer or mobile device language input settings.
To change keyboards, as you are tapping the space bar on your keyboard, swipe to the side. It is a toggle to change from one language keyboard to another. At least it works for me. I have an Android phone. This is after I added Greek (2 dialects of it) in my settings. I did both dialects because I wasn't sure which was the right one.