OK – what that bit of code is doing is listing all of your Pages. Assuming that your About page has an id of 2, the first task is to exclude it from the Pages menu and hide the Pages menu if you don’t have any Pages except About. So try changing:
<?php wp_list_pages('title_li=About'); ?>
to:
<?php if( wp_list_pages('title_li=&exclude=2&echo=0') ) wp_list_pages('title_li=Pages&exclude=2'); ?>
Then you’ll need to add in an essentially hard-coded link to your About page. So just above the new line, try adding:
<?php
$page_id = 2; // id of the About page
get_page( $page_id );
$page_title = $page_data->post_title;?>
<li class="about"<a title="<?php echo $page_title;?>" href="<?php echo get_page_link($page_id);?>"><?php echo $page_title;?></a></li>
The whole thing should look like:
<?php
$page_id = 2; // id of the About page
get_page( $page_id );
$page_title = $page_data->post_title;?>
<li class="about"<a title="<?php echo $page_title;?>" href="<?php echo get_page_link($page_id);?>"><?php echo $page_title;?></a></li>
<?php if( wp_list_pages('title_li=&exclude=2&echo=0') ) wp_list_pages('title_li=Pages&exclude=2'); ?>
At least that’s how it should work in theory, ??