[Plugin: AddThis] Exclude from posts in a certain category

Hello, I am looking for a simple function to run in my functions.php file that will allow me to set the default meta box to be in the checked state for posts in a certain category. This is as close as I have come.

add_filter(‘post_metabox’, ‘my_post_metabox’);
function my_post_metabox($checked)
{
if (in_category(’27’)){
$checked = “checked”;
}
return $checked;
}

http://ww.wp.xz.cn/extend/plugins/addthis/