What is the directory structure immediately above the directory containing the HTML chapters and the 'content.opf' file?
You usually see the structure you describe inside a .epub file (an .epub file is just a zipped up directory).
It may be that you have acidentally navigated inside the directory structure of a .epub file without realizing. In that case, try opening the .epub file in ADE.
If not, try packaging the entire directory as a zip file, rename it to .epub, and then try to open it with ADE.