You should be able to read from Firebird with no problems. Make sure the user you are connecting as has permissions to read all tables. Try doing a simple SQL query with any query analyzer logged as that user and see what happens. In case of doubt, test with an administrator account.
Even if you don't get any data in the wizard, what happens if you do a SQL SELECT * FROM Table ? I mean, just write the sentence in the script and reload the document, where obviously "Table" is an exisiting table in the database.
Hope that helps.