You should be able to go into your theme and add this link pretty easily.
If you open the header.php file for the Stylevantage theme, you will see a section of code like this:
<ul>
<li<?php if ( is_home() or is_archive() or is_single() or is_paged() or is_search() or (function_exists('is_tag') and is_tag()) ) { echo ' class="current_page_item"'; } ?>><a href="<?php echo get_option('home'); ?>">Home</a></li>
<?php wp_list_pages('title_li=&depth=1'); ?>
</ul>
That is your top navigation, it is simply a bit of PHP that makes a list, and that list forms your navbar.
With a minor modification,you can add links to this list:
<ul>
<li<?php if ( is_home() or is_archive() or is_single() or is_paged() or is_search() or (function_exists('is_tag') and is_tag()) ) { echo ' class="current_page_item"'; } ?>><a href="<?php echo get_option('home'); ?>">Home</a></li>
<?php wp_list_pages('title_li=&depth=1'); ?>
<!-- add this line -->
<li><a href="http://www.example.com/forum">Forum</a></li>
<!-- add this line -->
</ul>
It can be a little trickier to do this in phpBB, but there is a good support forum for finding answers, this answer may help you.