.centrediv 
{
    position:absolute;
    top: 50%;
    left: 50%;
    width:300px;
    height:300px;
    margin-top: -150px; /*set to a negative number 1/2 of your height*/
    margin-left: -150px; /*set to a negative number 1/2 of your width*/
    border: 1px solid #ccc;
    background-color: #FFFFFF;
    display:block;
}

.rightdiv
{
    position:absolute;
    top: 150px;
    right: 10px;
    width:300px;
    height:300px;
    border: 1px solid #ccc;
    background-color: #f3f3f3;
    display:block;
}