the dust you're seeing is most likely on the mirror - it won't effect the image quality because the mirror swings up to expose the CCD when you click the shutter button.
to clean the dust off the mirror you'll need a blower brush, which looks like this:
be VERY gentle and don't press hard at ALL. Just lightly lift the dust away from the mirror with the dust.
as for the viewfinder on your nikon, it's most likely an electrical connection problem that you're probably best off letting a pro handle.