You should not try and open the epub book from DropBox itself on the iPad as Dropbox does not (yet anyway) know how to find what app can open that file type.
Calibre2opds catalogs have been used with great success for libraries held in the 'Public' folder under Dropbox,. If you do this you have two options:
- Use an OPDS aware ebook reader app with XML version of the Calibre2opds catalog. Examples of such OPDS aware reading apps being Quickreader and Stanza.
- Use Safari in conjunction with the HTML version of the Calibre2opds catalogs. This should work for any reading app (e.g. iBooks) that registers itself as being capable of hanlong that file type.
Both methods are described in
Using calibre2opds catalogs section of the documentation. If you find the documentation unclear then mention where and an attempt can be made to clarify that area.