Welcome to Nikonians! If the dust is on the mirror box side of the viewfinder screen you might be able to clean it with a Giottos Rocket Blower. Just make sure that you don't touch the mirror or any part of the mirror box or finder screen with the Rocket Blower.
If the dust is above the finder screen things get more complex. You will have to carefully remove the finder screen without touching the mirror and use the Rocket Blower to dislodge the dust particles, then reinstall the screen. Unless you know what you are doing, I wouldn't recommend removing the finder screen. Good luck and Enjoy your Nikons!