My connection URL for DbVisualizer is
jdbc:DBF:////home/jerry/dbf/dbffiles
or
jdbc:DBF:////pathtodbffiles

Another problem was that although the References tab would make a graphical plot of the tables, SQL commands entered in that tab would fail. The reason for the failure is that the VFP DBF free tables should have UPPER case names and extensions, as should the column names too. Making sure that all DBF had UPPER case names, extensions, and column names, including CDX files, allowed the SQL command abilities to work.
JLK