I offer the same advice as Martin above.
I believe you have a Canon dslr in which case there is a diopter adjustment on the eye piece which is enough for most people to get a comfortable setting.  In cases where more adjustment is necessary there is an accessory available to give a wider range.

Refer to your manual for full instructions on setting the eyepiece diopter.
As a brief guide you need to switch off auto focus and point the camera at a blank wall. In the viewfinder you will see several red squares , the focus points, adjust the small wheel beside the eyepiece until these are in sharp focus.  

R

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