Open the Editor (cmd-E)
Click in the Text field and type in text.
To add second line of text click [1] or type // followed by the rest of the text (see example below)
Move text up or down [2] [3]
Click the down arrow at the end of the text field, for the dropdown menu of commonly used text (e.g. D.C. al Coda, D.S. al Coda, repeat times, rhythm options) that are also understood and interpreted by the Player.
cmd-S to Save (will automatically save your edits)
cmd-E to exit (older versions, cmd-W)
Some users have found ways to add lyrics using the text in the editor.
See their solution here.