Category limits

Hi There,

I’ve got trouble with the categories on my blog. I have more than there are currently shown on the site but for some reason it cuts the last two out and aligns everything to the left

http://docgeeks.com/

As you can see the grey block also stops there. Does this have something to do with ad space? I am such a beginner with this and I’m lost… Can someone help?

To show you what I’ve done here’s the CSS bit (probably wrong as well):

‘.main-navigation {
margin: left;
clear: both;
width: 100%;
background: #666;
}

.main-navigation a {
color: #000;
background: #888;
}

.main-navigation ul ul li {
background: #888;
}’