Open includes/templates/YOUR_TEMPLATE/common/tpl_main_page.php

for the left column, find:

Code:
<td id="navColumnOne" class="columnLeft" style="width: <?php echo COLUMN_WIDTH_LEFT; ?>">
<div id="navColumnOneTopWrapper">&nbsp;</div> 
<?php
 /**
  * prepares and displays left column sideboxes
  *
  */
?>
<div id="navColumnOneWrapper" style="width: <?php echo BOX_WIDTH_LEFT; ?>"><?php require(DIR_WS_MODULES . zen_get_module_directory('column_left.php')); ?></div><div id="navColumnOneBottomWrapper">&nbsp;</div></td>
<?php
}
?>
Add the portions highlighted in red.

for the right column find:

Code:
<td id="navColumnTwo" class="columnRight" style="width: <?php echo COLUMN_WIDTH_RIGHT; ?>">
<div id="navColumnTwoTopWrapper">&nbsp;</div>
<?php
 /**
  * prepares and displays right column sideboxes
  *
  */
?>
<div id="navColumnTwoWrapper" style="width: <?php echo BOX_WIDTH_RIGHT; ?>"><?php require(DIR_WS_MODULES . zen_get_module_directory('column_right.php')); ?></div><div id="navColumnTwoBottomWrapper">&nbsp;</div></td>
<?php
}
?>
add the portions highlighted in red.

save the file and upload to your server.