Short grace notes (Acciaccatura) appear as small notes with a stroke through the stem. Long grace notes (Appoggiatura) have no stroke. Both are placed before the normal-sized main note.
Create a grace note by dragging a grace note symbol from the Grace Notes palette to a regular note on the score. It's also possible to create a grace note by selecting a notehead and double-clicking a grace note symbol from the Grace Notes palette.
To add more than one grace note, drag successive grace notes on the notehead.
To add a chord of grace notes, enter the first one and select it, then use Shift⇑+ note names (C, D, E etc...)
If you want to change the duration of a previously created grace note, select it and choose a duration from the toolbar or enter with one of the keys 1 ... 9 (see Note Entry).