Tap the Annotate button on your right side menu. 

For simple text anywhere on your canvas, tap the Plain Text option.

Tap on your canvas where you want it to appear.

You'll then see the text box appear and you'll be able to type. Once finished, tap Done.

If you want to call out something on your canvas, use the text call out option.

Tap on an entity where you want to create a call out. 

Drag out and away from where want the text to appear. Then add your text and tap Done. 

How to add text to your canvas