in the webassist/themes folder, open the folder for the theme you are using
in your theme folder, open the Templates/page.dwt.php
in the default template, the code for the left menu is:
<div id="leftColumn1" class="ps_sidenav sidebar accordion">
<?php echo($WASideMenu->Body); ?>
</div>
you can replace the left menu with what ever you like by editing that:
<div id="leftColumn1" class="ps_sidenav sidebar accordion">
custom content in the left column
</div>