Add Custom Top Menu Navigation Link in Magento 1.9

First copy topmenu.phtml from

/app/design/frontend/rwd/default/template/page/html/

Save this file as topmenu.phtml to

/app/design/frontend/YOUR PACKAGE/YOUR THEME/template/page/html/

Edit this file within lines 38-40, paste the following:

<?php echo $this->getLayout()->createBlock('cms/block')->setBlockId('home-link')->toHtml(); ?>

To create:

<ol class="nav-primary">
<?php echo $this->getLayout()->createBlock('cms/block')->setBlockId('home-link')->toHtml(); ?>
<?php echo $_menu ?>
<?php echo $this->getLayout()->createBlock('cms/block')->setBlockId('post-menu-links')->toHtml(); ?>
</ol>

Save and re-upload this file.

Create a CMS > Static Block, named home-link enter the following into your block (Hide Editor, as needed):

<li class="level0 nav-1 parent"><a href='{{config path="web/unsecure/base_url"}}' title='Home Page' class='level0'>Home</a></li>

Note: I leave all of the class names consistent with 1.9. Feel free to adjust class names as needed.

Save your new Static Block.

Create a CMS > Static Block, named post-menu-links Enter the following into your block (Hide Editor, as needed):

<li class="level0 nav-1 parent"><a href="CUSTOMPATH-1.HTML" title="LINK TITLE #1" class="level0">LINKNAME #1</a></li>
<li class="level0 nav-1 parent"><a href="CUSTOMPATH-2.HTML" title="LINK TITLE #2" class="level0">LINKNAME #2</a></li>
<!--  and so on, as needed ...  -->

Save your new Static Block.

Be sure to clear the cache and refresh your browser.

Here is the proper way to add a cms page link to the main menu without using the event page_block_html_topmenu_gethtml_before.
This way you will be able to add an active state to it also.
So create an observer in one of your modules that can look like this:

public function addCmsPage($observer) 
{
    //your page identifier
    $cmsPageIdentifier = 'page_idendifier_here';
    //get the menu instance
    $menu = $observer->getMenu();
    //load the page
    $page = Mage::getModel('cms/page')->load($cmsPageIdentifier, $identifier);
    //if the page exists
    if ($page->getId()) {
        $active = false;
        // Get the full action name for the request.
        $fullActionName = Mage::app()->getFrontController()->getAction()->getFullActionName();
        //check if we are on the view page action for the required page
        if ($fullActionName == 'cms_page_view') {
             $pageId = Mage::app()->getRequest()->getParam('page_id', $this->getRequest()->getParam('id', false)); 
             if ($pageId == $page->getId()) {
                 $active = true;
             }
        }
        //build a new menu item node
        $data = array(
            'name' => $page->getTitle(),
            'id' => 'cms-'.$cmsPageIdentifier,
            'url' => Mage::getUrl('', array('_direct'=>$cmsPageIdentifier)),
            'is_active' => $active
        );
        $pageNode = new Varien_Data_Tree_Node($data, 'id', $tree, $menu);
        //attach the node to the menu
        $menu->addChild($pageNode);
    }
}