To add text and arrows to your diagram:

First, Select the "Label diagram"tool.

• Click to draw a label:
  
  • 
• Click + Drag to draw a label with an arrow:

In the property panel on the right, you can edit the text as well as add adjust various properties:

Notes

• To move the label, select the "Select & Move items" tool and drag the label or the arrow.
• To change where the arrow is pointing, click and drag the yellow diamond shapes.
• To hide the arrow head, toggle "Show arrow head"
• To hide the arrow completely, toggle "Show arrow"
• See this article on how to write chemical formulas, subscripts, units etc.

Alternative: Use the "Label" item

An alternative is to add the "Label" from "Decorative" section. Note this is designed for use on glassware, bottles etc. but can also be used for labelling the diagram itself.