Nikon makes a number of optional eyepieces for you Nikon DSLR cameras. I have three favorites - the one that comes with the camera, the right angle finder with 2 x magnification (DR-5) and the Nikon DK 17M.
Between the DR 5 Screw-in Right Angle Viewfinder and the Nikon DK 17M, I prefer the DK 17M due to its small size and ease of use. The DR-5 is best suited for situations when your camera is low and you need better access for making focus adjustments. The DM 17M works perfectly for situations when normal photography work is encountered such as doing macro work indoors.
How does it attach: The DK-17m basically screws onto the back of your camera body, slide the eyepiece cover switch down, then remove your old one by unscrewing it, then attach the new DK 17M - the entire process takes about one minute.
Is it compatible with my camera: Nikon makes it easy to determine what accessories are compatible with your camera. Go to the NikonUSA site, find your camera, then click accessories - within the accessories, you will find Eyepieces and Viewfinders.
What does it Magnify: The DK 17M magnifies the viewfinder by 1.2 times, making it easier to see fine details - GOOD for macro work. When attached and in use, the DK 17M provides very good coverage of the viewfinder and readout. However, you will need to "look around" to see edge to edge as the 1.2 magnification pulls the viewfinder image closer. This is not a problem for general photography use and is very useful for macro work or when working with manual focus lens.
-- As a disclaimer - many people find it easier to use the live view, the magnify the view - I prefer using the DK 17M optically.
I keep mine on most of the time and don't have any real problems using it daily, I will say if you are bird photographer, visually tracking birds in flight may be more difficult due to the magnification.
If you never tried an optional eyepiece and you do macro work I recommend you give it a try, the current cost is about $50.00, the added magnification is a plus for critical focus, especially for manual focus on specific areas.