Because it doesn't work straight out of the box, if you just load the control it will not display anything since there is no page_load implementation in it.
You must tell ForumPost what forumpost that should be displayed, look in the code behind.
So the steps you must take is...
Get a ForumPost object by calling the appropiate function and you will need a forumpostID to do this, you must implement the functionality that calls DataBind to be able to show data in the ForumPost.ascx control if you are going to use it.
