if a <div> has the class mkteditable it should show up in the landing page designer. Don't forget every editable <div> must have a unique ID (id="somename").
When this is set you are able to alter some basic elements in the class, like height, width, and so.
P.s. don't expect a WYSIWYG-editor to show up. You are responsible for dragging in landing page elements (e.g. image, rich text, or form) on to the page. The teamplte only defines the structural areas such as backgrounds. The landing page lements do not interact with the editable <div>'s.
If you already know all of the above, something else is wrong. I once experienced this, and I just couldn't find out why. Eventually I found out it had something to do with missing default elements in the landing page template HTML. Al lof this is explained in the advanced landing page template course (the one with 5 videos).