1. Open the Editor.
  2. Click in the Text field.
  3. Use // to add second line of text.
  4. When you click on Save, it will automatically save all your edits.
If you click on the down arrow at the end of the text field, you will see a list of commonly used texts (e.g. D.C. al Coda, D.S. al Coda, repeat times) that are also understood and interpreted by the Player. 
You can add any text, simply type in text field at the current cursor position.
Use // to add a second line of text.
The two arrows will allow you to move the text up or down.

Some users have found ways to add lyrics using the text in the editor. 
See their solution here.