Sorry - I *just* saw this. When people post to ask me questions about the plugin, they usually put it in the topic name - when they do, i emails me the questions. otherwise you have to rely on when I have time to come and check :)
So if I'm reading this correctly, you're getting the instruction to show up on a particular post (with an ID) but you want it to show up on all posts? This is actually covered in the readme file. (to be specific, item #4, "Page Name" section describes how to do this in detail. Specifially here: "copy everything after the last "/" but before the "?" (in this example: "post-new.php") and paste it into this field") So using the URL you gave as an example, this:
post.php?post=28&action=edit
Will cause the instruction to show up only when you are editing a post with the ID of 28. If you want it to show up when you're editing ANY post, you just have to enter in:
post.php