Hi Madsy,
Notation mode uses the 'grand stave' approach for entering music. That is, when notes are above middle C, they will be written on the treble and if they are below middle C they will be written on the bass clef.
It's possible to change the point in the staff that this split occurs but it is limited. The setting can be found under Notation Mode options. It's called "Clefts split at" and there is a "C4" entered. This is the default. If you change it to F4, F3, G4, etc., you'll see the impact it has. I suspect that this is not what you are after.
Note: The C4, F3, etc., is a midi indentification for notes. C4 = middle C, C5 = one octave above middle C, C3 = one octave below middle C. G4 = the G above middle C, etc.
Can you outline what you are hoping to achieve by entering the piano score? I might have some ideas that will help once I know what you'd like to accomplish.
Regards,
Noel.