I'm using the Reveal jQuery plugin to display a modal window (http://www.zurb.com/playground/reveal-modal-plugin). Everything works fine, but I have one problem. The modal window is displayed at the bottom of the page and I want it in the middle of the page.

You can see an example at: www.kinderwagenstekoop.nl/kinderwagens
(below the list items and above the small pictures click on the link 'Bestellen')

I can't solve this css issue. Tried everything. I hope someone can help me with this.

CSS of the modal window:

.reveal-modal-bg {
position: fixed;
height: 100%;
width: 100%;
background: #000;
background: rgba(0,0,0,.8);
z-index: 100;
display: none;
top: 0;
left: 0;
}

.reveal-modal {
visibility: hidden;
top: 100px;
left: 50%;
margin-left: -300px;
width: 520px;
background: #eee url(modal-gloss.png) no-repeat -200px -80px;
position: absolute;
z-index: 101;
padding: 30px 40px 34px;
-moz-border-radius: 5px;
-webkit-border-radius: 5px;
border-radius: 5px;
-moz-box-shadow: 0 0 10px rgba(0,0,0,.4);
-webkit-box-shadow: 0 0 10px rgba(0,0,0,.4);
-box-shadow: 0 0 10px rgba(0,0,0,.4);
}

.reveal-modal p {
color: #474747;
}

.reveal-modal.small { width: 200px; margin-left: -140px;}
.reveal-modal.medium { width: 400px; margin-left: -240px;}
.reveal-modal.large { width: 600px; margin-left: -340px;}
.reveal-modal.xlarge { width: 800px; margin-left: -440px;}

.reveal-modal .close-reveal-modal {
font-size: 22px;
line-height: .5;
position: absolute;
top: 8px;
right: 11px;
color: #aaa;
text-shadow: 0 -1px 1px rbga(0,0,0,.6);
font-weight: bold;
cursor: pointer;
}
/*