I am reasonably certain that the Dear Author article was against a very much earlier version of Calibre2ops - probably one of the 1.x releases.
You put the Calibre library folder (where the metadata.db file is located) as the Database folder in calibre2opds. I am assuming from the fact that you mention Dropbox this will be a folder under Dropbox. This is the only mandatory option that the user has to set. If you leave everything else at defaults then the catalog is created as the _catalog folder inside that, and the index.hml file is inside the _catalog folder.
Have you looked at the calibre2opds Online User Guide available either by looking in the documentation section at
http://calibre2opds.com, or via the program's Help menu. That should answer most questions, and if not we would like to know where the documentation is deficient so it can be improved.