This search:
http://wordpress.org/search/sidebar+on+pages+in+default+theme?forums=1
found, among others, this:
http://wordpress.org/support/topic/152249?replies=5
Essentially you change the wp-content/themes/default/single.php file to display the sidebar:
Add this line:
<?php get_sidebar(); ?>
right before this line:
<?php get_footer(); ?>
——
and change this line from:
<div id=”content” class=”widecolumn”>
to this:
<div id=”content” class=”narrowcolumn”>