Lookup "ONLINE STORE" in your html file and change it to: <a href="URL">ONLINE STORE</a> Code (markup): where URL is the url of the other html file.
http://wordpress.org/extend/plugins/quick-pagepost-redirect-plugin/ that will do exactly what you want
instead of getting that menu from post or from adding page you need to just give direct link in header.php there you can find navigation ... such as <li><a class="<? echo (is_home())?'on':''; ?>" href="<?php echo get_option('home'); ?>/">Home</a></li> and you can add <li><a href="../path">ONLINE STORE</a></li>