First, make sure you're using the latest version of our Wordpress plugin. Then go to the AddThis settings page and click click "additional style options"

Then click the check box next to "Custom Button"

Then, enter the code you want to use, making sure to put it between <div> tags. For example:
<div class="addthis_toolbox addthis_default_style "> <a class="addthis_button_google_plusone"></a> <a class="addthis_button_facebook_like" fb:like:send="true"></a> </div>