From the skininfo.html thats included in the downloadable zip:

Calendar can be un-commented.
Styles for calendar are in the style sheet

Now that you have read that, look inside _main.php for where it IS commented:

<!-- =================================== START OF SIDEBAR =================================== -->

<?php if( $disp != 'single' ) { ?>
<div id="bSideBar">

<div class="bSideItem">
<p><?php $Blog->disp( 'longdesc', 'htmlbody' ); ?></p>
<p class="center"><strong><?php posts_nav_link( ' | ', '< '.T_('Previous'), T_('Next').' >' ); ?></strong></p>
<!--?php next_post(); // activate this if you want a link to the next post in single page mode ?-->
<!--?php previous_post(); // activate this if you want a link to the previous post in single page mode ?-->
<?php // require( dirname(__FILE__).'/_calendar.php' ); // uncomment this line for the calendar ?>

See the bottom line. Remove the the leading // like the skininfo says.