WP-PageNavi is for the bottom so instead of your visitors just clicking next and previous to get more articles, you have more pages (like on my site at http://wpaddict.net/ )
What type of navigation are you trying to put up? A horizontal navigation?
If horizontal - here is just an example in css of one. I do not see it in your layout other than your pages in a sidebar widget.
#mainmenu ul{
margin: 0;
padding: 0;
list-style-type: none;
z-index:1000;
}
HTML
<div id="mainmenu">
<ul>
<li class="first <? if(is_home()) echo 'current_page_item'; ?>"><a href="<?php echo get_option('home'); ?>/">Home</a></li>
<?php wp_list_pages('title_li=&depth=1'); ?>
</ul>
</div>
It is the list-style-type: none; that allows a horizontal navigation instead of vertical.
Tip: all your style sheets need to be before the ending head tag, not within the body of your layout.