I can't align two shapes properly in Illustrator CS6

I'm a new member of this family. I really like this website because I was helped by it :) anyway I'm facing a problem for some days. When I'm trying to align two shapes such as a RECTANGLE and a CIRCLE, I can never align them left/right/up/down :( Is there anybody who can help me to fix this problem...??? I'll be really grateful to him/her :)

Thanks!