Hi,
One way how you can add a pop up message after the user click "Add" buton is to modify the following file
~/CMSModules/MessageBoards/Controls/Messages/MessageEdit.ascx.cs. In this file around line 222 there is method called btnOk_Click and you can add at the beginning of this method something likes this:
Response.Write("<script>alert('Successfully added');</script>");
I hope this will help you.
Best Regards,
Roman Konicek