To set the title of the item being shared you can do it via the addthis_share JavaScript variable, like this:
<script type="text/javascript>
var addthis_share = {
title: "The Title to Share."
}
</script>
Or you could define it in the toolbox div:
<div class="addthis_toolbox addthis_default_style " addthis:title="The Title to Share">
When sharing to certain services, notably Facebook, they will scrape your page. You'll have to use meta tags to define the title you'd like to share for those services.