Take a look at this demo: http://jsfiddle.net/dj2fu/
To achieve the desired result, you'll need to make a few adjustments.
Firstly, eliminate the <br>
tags from your code.
Next, add the following CSS styles:
html, body {
min-height: 100%;
position: relative;
height: 100%;
padding: 0;
margin: 0;
}
#divMainMenu {
margin: auto;
height: 100%;
width: 100%;
display: table;
}
#divMainMenu input[type="button"] {
display: block;
width: 50%;
margin: auto;
height: 20%;
margin-bottom: 2.5%;
padding: 0;
margin-top: 2.5%;
}