piano/guitar music

Hi.. About 6 months ago I finally gave up my pen and manuscript paper and have really enjoyed writing legible (I am a doctor!) parts for my friends and me to play now I have semi-retired. I have also really enjoyed adapting scores from your library to adapt for the instruments my friends play- like the Marche Militaire I uploaded.

But I have problems writing music for piano or for my friend on classical guitar. It seems to be impossible to write notes at other beats in the bar apart from the melody line/ highest part. I can add notes in the same time below the upper part, but no rests and then fill notes as one would do for piano or classical guitar music. Are you altering musescore to enable this, or have i missed something?. I do not want to put them on separate staves, i want to create standard music