The two main places the sand could be is on your mirror or on your focus screen. I'd suggest using a bulb blower (e.g., RocketBlower), and not physically touching anything in the camera (e.g., using a brush or microfiber or something). Whatever you do, don't use canned air. Simply taking the lens off the camera will show you the mirror. The focus screen is the "ceiling" of the camera chamber. Basically hold the camera face down, so anything you shift with a few puffs will fall down and out of the camera, rather than getting shoved around somewhere else. This sensor-cleaning video from LensRentals demonstrates using a blower (around 0:27). Obviously, you don't want to be in the sensor cleaning mode and exposing the sensor when you try to puff your sand grain out. :)

Or you can just live with the dot in your viewfinder. I did that after failing to remove the specks and scratching my focus screen :(. Until years later when I had to bring my XT in to Canon Irvine for a busted power board, at which time they replaced my focus screen for free (still had to pay for the powerboard replacement, though).