you havent pasted it as it is right ?

// ------------------------- "Post Footer" CONTAINER EMBEDDED HERE --------------------------
// Display container and contents:
skin_container( NT_('Post Footer'), array(
// The following params will be used as defaults for widgets included in this container:
'block_start' => '',
'block_end' => '',
) );
// ----------------------------- END OF "Post Footer" CONTAINER -----------------------------

i assume you know you had to add php tags to it so that it'd look like this:

<?php // Display container and contents:
skin_container( NT_('Post Footer'), array(
		// The following params will be used as defaults for widgets included in this container:
		'block_start' => '',
		'block_end' => '',
	) );?>

other than that i couldnt guess of any other reasons why it would not render

to be precise, after adding the above green lines, you will have a new container called 'Post Footer' under your skin settings (if you still dont have it there go to blog settings > skins > manage skins > reload containers), than the final step add the widget into this container

you need to add a container on post foot, as tilqicom said ;)

Thanks, but fpr some reason I am not able to make it work.

The code looks like this after I have added the new container. I really don't know how to proceed. I'd appreciate a lot if somebody finds out where the problem is.

<?php
// Link to comments, trackbacks, etc.:
$Item->feedback_link( array(
'type' => 'feedbacks',
'link_before' => '',
'link_after' => ' &bull; ',
'link_text_zero' => '#',
'link_text_one' => '#',
'link_text_more' => '#',
'link_title' => '#',
'use_popup' => false,
) );
?>
// ------------------------- "Post Footer" CONTAINER EMBEDDED HERE --------------------------
<?php
// Display container and contents:
skin_container( NT_('Post Footer'), array(
// The following params will be used as defaults for widgets included in this container:
'block_start' => '',
'block_end' => '',
) );
?>
// ----------------------------- END OF "Post Footer" CONTAINER -----------------------------
<?php // ------------------------- "Post Footer" CONTAINER EMBEDDED HERE --------------------------
$Item->edit_link( array( // Link to backoffice for editing

after adding the above lines, you will have a new container called 'Post Footer' under your skin settings (if you still dont have it there go to blog settings > skins > manage skins > reload containers), than the final step add the widget into this container

that part is ok.now you should proceed with adding the widget.
have you done these steps ?

Shameless plug here :) If you want to put the sharethis thing on every post, there is already a plugin which does precisely that: http://forums.b2evolution.net/viewtopic.php?t=16263

Hello, I installed the plugin, added container in index.main.php (Asevo theme), added the style to the CSS file, yet it is not appearing still.

Any thoughts?

index.main.php:

<p>
				<?php
					// Link to comments, trackbacks, etc.:
					$Item->feedback_link( array(
									'type' => 'comments',
									'link_before' => '',
									'link_after' => '',
									'link_text_zero' => '#',
									'link_text_one' => '#',
									'link_text_more' => '#',
									'link_title' => '#',
									'use_popup' => false,
								) );
				 ?>
				<?php
					// Link to comments, trackbacks, etc.:
					$Item->feedback_link( array(
									'type' => 'trackbacks',
									'link_before' => ' &bull; ',
									'link_after' => '',
									'link_text_zero' => '#',
									'link_text_one' => '#',
									'link_text_more' => '#',
									'link_title' => '#',
									'use_popup' => false,
								) );
				 ?>
                 <?php
// ------------------------- "Post Footer" CONTAINER EMBEDDED HERE --------------------------
// Display container and contents:
skin_container( NT_('Post Footer'), array(
		// The following params will be used as defaults for widgets included in this container:
		'block_start' => '',
		'block_end' => '',
	) );
// ----------------------------- END OF "Post Footer" CONTAINER -----------------------------
?>
			</p>

style.css:

div.sharethis {
    display: inline;
    margin-left: 2em;
}

When I edit a post, I see the plugin in the list, and it is check (and greyed out). Looks like it installed correctly, and I am suspecting it's my CSS or theme. Any help is appreciated.