Use the Add Instrument dialog to add any number of new staves to your music. Each staff is of a particular type (e.g. drum staff, grand staff), it has a clef, a key, an instrument and a name. Collectively, these are all referred to as a part. The Add Instrument dialog is really used to add parts to your music.
Select the main tab "Score" "Add Instrument..." to display the Add Instrument dialog. Click on the image below for details on each control. When you are ready to add the staves, press the OK button; or, press the Cancel button to abort the operation.
Click on areas of the image below for more information: