Rename the ztd at the end of your creation to zip. Extract all the files.
Go into your ucs file with Notepad and find the section with the heading: [Characteristics/Integers]
At the end of the list, add the line:
walkable = 1
Save and exit Notepad. Go out into the directory with the folders Scenery and Objects. Zip up both those folders in the same zip file. Rename the zip at the end of the file to ztd.
That's all.
