Layout appears different from macbook and iMac

Hello,
I’ve been working on my site form a macbook air. Everything seemed settled, my layer slider looked perfectly centered. Then I went on my iMac and suddenly everything appears in different places! My layer slider animation is not centered anymore (and I specified on the settings it had to be centered) and in all the pages my site it seem as if there was a left sidebar, which is empty thought. I have a right sidebar in all my pages, but here, on the iMac, there is a huge blank space on the left. How can I do so that what I see, when I work on the layout on my macbook air, is the same that everybody, with different computer, see?
Thank you very much!

Beatrice