You should just be able to select a key signature and insert it into the staves. Make sure that you have the "show pallet" option checked in the "Window" drop down menu and it should appear at the bottom of your score. There is a symbol that has 3 sharps and 4/4. Click on it to expand it and you will see an icon with 3 sharps. Click on that and choose D for your tonic and Major for your mode.
I hope this is what you were looking for.
Best of luck,
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