The URL you need in Stanza will need to be one that is custom to your DropBox account.
I am not a DropBox user myself, but the tutorial
here on setting up DropBox to work with Calibre2Opds generated catalogs may give you the clue to the missing information you need about the URL to be used to access DropBox.
You may find you also want to use the
Calibre2opds tool to give you Stanza friendly catalogs.