Hi,
I've just downloaded the Ahimsa theme in order to take a look for you.
There are a few options for you here, depending on what you would like the subheading to look like.
For each of the following examples, I have pasted the code in the Main Index Template (index.php), on the line immediately after <div class="entry">.
The first example styles the subheading a little like the datetime / author information, but is left aligned:
<?php if (function_exists('the_subheading')) { the_subheading('<h2><span class="capsule">', '</span></h2>'); } ?>
If however you just wanted to display the subheading in a larger, bold, black font, you could instead use:
<?php if (function_exists('the_subheading')) { the_subheading('<h3>', '</h3>'); } ?>
You could have it displayed as part of the main title (immediately afterwards) by using the adjusting the title tag so it looks like:
<legend class='title'>
<a href="<?php the_permalink() ?>" rel="bookmark"
title="Permanent Link to <?php the_title(); ?>"><?php the_title(); ?>
<?php if (function_exists('the_subheading')) { the_subheading('', ''); } ?></a>
</legend>
I think the second option looks a little cleaner personally, but it's up to you really :)
Also, you will have to do something similar to entry.php in order to get it displayed on the individual posts and pages.
Hope this helps!