Admin widget on the Write Post page for inserting HTML encodings of special characters into the post.
You can't. The plugin only inserts the HTML character encodings into the post body. However, you can certainly hover over the character you want to use to see the HTML encoding for it (it'll start with an ampersand,
&, and end with a semi-color,
;) and type that into the field.
It is in the sidebar, most likely at the sidebar's bottom.