Find:

"<div class="post-content">" in index.php

Add the following:

<div class="diggit"><script type="text/javascript">
digg_url = '<?php the_permalink() ?>';
digg_title = '<?php the_title(); ?>';
</script>
<script src="<a href="http://digg.com/tools/diggthis.js">http://digg.com/tools/diggthis.js</a>" type="text/javascript"></script>
<iframe src="<a href="http://reddit.com/button?t=3&amp;url=<?php">http://reddit.com/button?t=3&amp;url=<?php</a> the_permalink() ?>" frameborder="0" height="75" scrolling="no" width="75"></iframe>
</div>

In the style.css document add:

.diggit {
float:right;
margin-top:10px;
margin-left:15px;
}

Should work for most skins