From my understanding, the "Editable Region" philosophy does not work on landing pages, only emails.
Here is what you should do to accomplish this task. Go to your form in Design Studio. On the left hand tree right click your form name and choose "Embed code". Copy the block of code it gives you in the pop up box. Now you can paste this code into your landing page template exactly where you want it to reside. You may need to adjust the positioning with CSS.
Hope this helps!
Are you using a "Free-form" landing page or "Guided" landing page template?