MuseScore -- Index -- Enter Notes

Notes are entered in <Note Edit> mode. First select a note or rest as start position for note entry. When entering notes you are always replacing existing notes or rests. So the duration of a measure does not change.

<N> enter <Note Edit> mode.
<Escape> leave <Note Edit> mode.

After entering <Note Edit> mode you should select the duration of the note you want to enter by selecting a value from the note palette or by pressing:

<Alt+1>   1/4 note      <Alt+2>   1/8 note
<Alt+3>   1/16 note <Alt+4>   1/32 note
<Alt+5>   1/64 note <Alt+6>   1/1 note
<Alt+7>   1/2 note

Notes are entered by pressing
<C D E F G A B>:
<Space> creates a rest
<C D Space E>:
Notes are added to chords by pressing <Shift+Notename>
<C D Shift+F Shift+A E F>:
Beams are created automatically
<Alt+1 C D Alt+2 E F G A>: