Oracle supports multiple naming methods to resolve Connect Identifiers. Connect only supports the Oracle Local Naming Method, which uses aliases defined in the tnsnames.ora configuration file on the Oracle client machine.This file is expected to reside in the <oracle_home>/network/admin directory, where <oracle_home> denotes the directory where Oracle is installed, or in the directory pointed to by the TNS_ADMIN environment variable. The Task Editor always reads the list of available databases from tnsnames.ora to automatically populate the list of databases in the Database Connection dialog. The file has to be formatted according to the Oracle documentation on syntax rules for configuration files. Otherwise, Connect may not be able to read the file correctly, resulting in an empty or partial database list in the Database Connection dialog. Verify that TNSNAMES is listed as one of the values of the NAMES.DIRECTORY_PATH parameter in the Oracle Net profile sqlnet.ora. The TNSNAMES field indicates that local naming is enabled.
If TNSNAMES is not listed as one of the values of the NAMES.DIRECTORY_PATH parameter in sqlnet.ora, run Oracle Net Configuration Assistant or Oracle Net Manager to add local naming method and the Oracle databases you want Connect to connect to. The configuration utility updates the Oracle Net profile, sqlnet.ora, located in <oracle_home>/network/admin.