In your stylesheet.css find the following:

/*wrappers - page or section containers*/
#mainWrapper {
margin: 0; /* Add this */
background-color: #ffffff;
text-align: left;
width: 1003px;
vertical-align: top;
}

#headerWrapper, #contentMainWrapper, #logoWrapper, #cartBoxListWrapper, #ezPageBoxList, #cartBoxListWrapper ul, #ezPageBoxList ul, #mainWrapper, #popupAdditionalImage, #popupImage {
margin: 0em;
padding: 0em;
}

#navColumnOneWrapper, #navColumnTwoWrapper, #mainWrapper/*Delete This*/ {
margin: auto;
}

Follow the highlighted instructions and that should solve the problem