I assume you mean you want to see the notes shown as flats and not sharps. Unfortunately you can’t, it only shows sharps. It’s often been requested to let us specify whether sharps or flats are used. But so far nothing, which seems surprising since I can’t imagine it being difficult to implement in addition to being musically correct.
The only place that Cubase will show the black keys as flats is in the Score Editor. Everywhere else it shows the notes on black keys as sharps even if they are properly flats. So the sharp keys display properly in the Key Editor but the flat keys do not. This is a problem that should have been fixed long ago.
However if you use the chord track you can set a scale/key there. I recommend deselecting “Automatic Scales” in the inspector of the chord track as it just inserts lots of extraneous scales if you use any chords with a note in them that is not a note in your intended scale. Once you have a scale and some chords set on the Chord Track, if you are in the Key Editor you can color code the notes based on the scale. Use the Event Colors drop-down menu (the one with 4 small colored squares) to select Chord Track. Now the notes will be color coded by A) In both the chord and scale, B) In the scale but not the chord, C) In the chord but not the scale, and D) Not in the scale or the chord.
While this is not a substitute for having notes properly show up as flats, it is useful.