It would help if you could show a live version of the site.

I noticed you have a double sidebar, that might cause some trouble.
Instead of using: <div align="center">Nav</div>, it's better to set:
.nav {
text-align: center;
}

To add a footer just add <div class="footer"></div> to your html and use the following CSS:
.footer {
clear: both;
}
Assuming you want the footer to be placed under your content..