You got some good advice above !

If you can see dust in the viewfinder it is most likely on the focussing screen. Live with it until the next service or get a Canon service center to clean it. Mirrors, focussing screens and prisms inside your camera are very easily damaged and any attempt by someone not experienced in cleaning these parts is likely to make matters far worse.  

Canon do the first clean for free and using any other method other than that explained in the instruction manual for cleaning will invalidate your warranty.

And every one said, 'If we only live,
We too will go to sea in a Sieve,---
To the hills of the Chankly Bore!'
Far and few, far and few, Are the lands where the Jumblies live;
Their heads are green, and their hands are blue, And they went to sea in a Sieve.

Edward Lear
http://www.nonsenselit.org/Lear/ns/jumblies.html