To search for dictionary files, zbedic usues Qtopia method Global::findDocuments, which "Finds all documents in the system's document directories...". Therefore, if a dictionary file is in the directory, which Qtopia does not consider as the right place for the document files, zbedic will not find it. I guess that the method does not search in "/opt/QtPlamtop/" directory.
An alternative method is to click on the dictionary file in the documents tab. This should always work. New zbedic has an additional section in the help that explains how to install dictionary files.