Nathan,
I don't think I understand this portion: Now click your bookmarklet to expand the Custom Head HTML field to full screen. Click again once you're finished editing.
I opened the landing page action and then clicked on edit meta tags, after that I am not sure what you mean "click your bookmarklet to expand..." could you please help me understand this?
The new product release scheduled for the end of October 2015 builds in the functionality to search for Landing Pages by URL. Here's the release notes, it's the bottom of the list: