Showing "Explanation or submission guidlines"
The "Explanation or submission guidelines" set on the edit/create content type form is actually displayed using the "System help" Block. So you just need to add the "System help" block to a region in your custom theme and it will be displayed there as long as you are actually printing the region in your theme.

I figured this out using the Theme developer module, which really is an extremely useful module for this kind of stuff. When you enable Theme developer, it adds a checkbox labeled "Themer info" to the bottom of each page. Check that box then click on any element in the page and you'll see information about how that element was themed. For example, enable Theme developer and use one of the default themes where the "Explanation or submissions guidelines" content is displayed correctly. Check the "Themer info" box and click on the submissions guidelines text, it will popup a box with information about how the submission guidelines where themed, like the image below. This should give you all the information you need to figure out how to theme something, or at least where to start looking.



NOTE: The Theme Developer module can actually end up breaking parts of your theme when enabled. It injects lots of &lt;span&gt;s to do it's magic. If something looks broken in your theme, disable Theme Developer and check again. Also, don't enable Theme Developer on a live site.