Either if the grain of sand is on the view finder or on the sensor, it might help to try to clean the camera. A tool I found particularly helpful in recent years was the “blower” which can be found in different shapes and colours from different manufacturers. Using it, you should be able to clean either with a soft air blow.
If you don't feel safe about fiddling with your camera in a way you do not think you are supposed to, it might be better to give it to a camera shop—many, if not most of them offer (sensor) cleaning for DSLRs for an affordable price.
As ElendilTheTall already mentioned in his comment: Don't worry too much about polluting your camera even more. Just make sure you are in a well-lit (so you can see the dust and grains) room without too much wind around and you should be safe.
Step 3 in this guide: Canon sensor cleaning: remove dust in 4 steps shows the tool I was referring to earlier. Although the guide is aimed at Canon users, most of the steps apply to Nikon—or any DSLR cameras for that matter—as well.