With Nightingale, the easiest time to place an accidental on a note is when the note itself is being entered.  This approach takes some getting used to, but is fast and easy once mastered.

• First, enter and position a note as required (see help on Entering Notes & Rests if you need guidance on how to do this) but do NOT release the mouse button. Then, if you want an accidental, hold down the Shift key and slide the mouse and up and down to get the correct accidental.  Be sure you release the mouse button before the Shift key.

• Accidentals on existing notes can be changed in a similar way. To add or change an accidental, first select the note, then click on it again with the Shift and Option keys both held down.  Hold the mouse button down and slide the mouse and up and down to get the correct accidental.  Be sure you release the mouse button before the Shift key if you want the accidental to appear. On the other hand, to REMOVE an accidental, drag it up or down without the Shift key, then just drag it back where it was.

• Musically unnecessary accidentals can be removed en masse with 'Delete Redundant Accidentals' (in the Notes menu).