There are a few ways to enter notes in the Notation Editor. You can click on the Piano Roll keys, or you can enter notes directly from your computer’s keyboard.
Notes from the keyboard: the keys [A] to [G] will add/change the note on the staff in/to the corresponding pitch (A-G).
Enharmonic spelling / accidentals: You can change the enharmonic spelling (adding accidentals) with [Shift]+[E] (E for enharmonic).
With [Shift]+[F] you will get a courtesy or cautionary accidental (F just comes after E).
Intervals / Triads / Chords: you can add intervals, triads or chords by using the number keys along the top of your keyboard. The  makes a third, the  makes a fourth, and so on.
[Shift]+[„number“] will give you the interval down.
Note that the interval starts from the highest note for intervals going up, and the lowest note for intervals going down. You can select individual pitches in the chord using [Shift]+[↑] or [Shift]+[↓].
This is similar to how modifiers like dynamics are selected. Once you have the pitch selected, you can do all other changes, like the enharmonic spelling.
There is currently no way to remove a single pitch from an interval, triad or chord. If you want to collapse an interval, triad or chord, just type a letter A-G on the keyboard, and it will be replaced with a single note.
Notes from the Piano Roll: You can simply add/change notes by clicking on the keys of the Piano Roll.
The octave of the note is based on the clef, so for treble clef, the „C“ is middle „C“. You can change the octave of the note, and move the pitch up and down.
The first group of buttons to the right of the Piano Roll affect the pitch. The icons for „Octave up“, „Octave down“ and „Interval/Triad/Chord“ will stay enabled when you click on them. To disable them, just click on them again.