MuseScore -- Index -- Enter Notes

Notes are entered in Note entry 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 entry mode .
Escape leave Note entry mode

After entering Note entry 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   Quarter note (crochet)      Alt + 2   Eighth note (quaver)
Alt + 3   1/16 note (semiquaver) Alt + 4   1/32 note (demi-semiquaver)
Alt + 5   1/64 note (semi-demi-semiquaver) Alt + 6   Whole note (semibreve)
Alt + 7   Half note (minim)

Notes are entered by pressing
C D E F G A B C :
Notes: C, D, E, F, G, A, B, C
Space creates a rest
C D Space E :
C, D, rest, E
Notes are added to chords by pressing Shift +Notename
C D Shift +F Shift +A E F :
C, D minor triad, E, F
Beams are created automatically
Alt +1 C D Alt +2 E F G A :
Quater notes: C, D, Eight notes: E, F, G, A