Since I don't know that much about your particular problem, I'd start start by looking into addthis.toolbox('.yourClass');
If you have a typical toolbox like this...
<div id="myToolbox" class="toolbox addthis_toolbox addthis_default_style ">
<a class="addthis_button_facebook" style="cursor:pointer"></a>
<a class="addthis_button_twitter" style="cursor:pointer"></a>
<a class="addthis_button_email" style="cursor:pointer"></a>
</div>
Once your ajax content has finished loading into the dom you could do the following...
addthis.toolbox('#myToolbox');
However, watch out for this!
Don't put your like button inside your toolbox because when you call the addthis.toolbox method it will create a duplicate like button iframe for some reason. It must be a bug but it took a couple years off my life. Instead you should put it in its own toolbox containing div and call the method on it as well.
In the case of multiple toolboxes
you should probably use a class instead. See the following code for the final example.
html
<div class="toolbox">
<a class="addthis_button_facebook_like" fb:like:layout="button_count" addthis:userid="myCompany"></a>
</div>
<div class="toolbox addthis_toolbox addthis_default_style ">
<a class="addthis_button_facebook" style="cursor:pointer"></a>
<a class="addthis_button_twitter" style="cursor:pointer"></a>
<a class="addthis_button_email" style="cursor:pointer"></a>
</div>
javascript:
//on complete of ajax load
addthis.toolbox('.toolbox');
addthis.toolbox($('.addthis_toolbox')[0])
(in jQuery) – Foreplay