You can insert, add or destroy complete measures using these buttons in the topmost part of the palette.
You can change the time signature by pressing this button from the palette.
This will bring up the Time Signature dialog where you can easily change your signature to anything you like.
You can also change the beaming pattern of eighths associated with this time signature on the same dialog.
You can change the key signature by pressing this button from the palette.
This will bring up the Key Signature dialog where you can easily change your signature of the measure the cursor is on.
To change the clef of your staff, bring up the Clef dialog with this button from the palette.
In the Clef dialog, you can choose from Treble, Bass and C Clefs for each staff line.
You can also change the Ottavia of your Clef there, making the staff sound one or two octaves lower or higher.
All the lower part of your window is occupied by the Arranger.
This gives you an overall vision of your score, with a row for each instrument.
You can toggle visibility of tracks in your score using the (V) button.
You can also tweak Solo/Mute, Volume and Pan of your tracks in there.