Here's a very simple way to show the AddThis code to the bottom of your entries. Sign up for a free account here. Open ADV_DYNA_SHOWENTRY and find the following:

Code:
            <vb:if condition="$entry['showsignature'] AND $entry['signature'] AND $vba_options['dyna_entry_sigs']">
                <blockquote class="signature restore"><div class="signaturecontainer">{vb:raw entry.signature}</div></blockquote>
            </vb:if>
After it add your Addthis code, with a <br> at the start:

Code:
<br><!-- AddThis Button BEGIN -->
<div class="addthis_toolbox addthis_default_style ">
<a class="addthis_button_preferred_1"></a>
<a class="addthis_button_preferred_2"></a>
<a class="addthis_button_preferred_3"></a>
<a class="addthis_button_preferred_4"></a>
<a class="addthis_button_compact"></a>
<a class="addthis_counter addthis_bubble_style"></a>
</div>