It is probably a configuration issue.
Things to check:
– You have created an ODBC connection to your MySql database and you have tested it from outside Oracle, and it is working well.
– Your Oracle global_names parameter is set to false (init.ora)
– You have configured the hetergeneous services. (This consists of creating an initodbc.ora)
– You have configured the listener to point to the heterogeneus service (listener.ora)
– You have created a new connection in your tnsnames.ora file
– You have restarted your listener service.
– You have created a database link