This is from the documentation -
Database: Third Party JDBC Drivers
The Third Party JDBC Drivers pane specifies drivers to be used for connections to third-party (non-Oracle) databases, such as IBM DB2, MySQL, Microsoft SQL Server, or Sybase Adaptive Server. (You do not need to add a driver for connections to Microsoft Access databases.) To add a driver, click Add Entry and select the path for the driver:
For Microsoft SQL Server or Sybase Adaptive Server: jtds-1.2.jar, which is included in the jtds-1.2-dist.zip download
To find a specific third-party JDBC driver, see the appropriate website (for example, http://www.mysql.com for the MySQL Connector/J JDBC driver for MySQL, http://jtds.sourceforge.net/ for the jTDS driver for Microsoft SQL Server and Sybase Adaptive Server, or search at http://www.teradata.com/ for the JDBC driver for Teradata). For MySQL, use the MySQL 5.0 driver, not 5.1 or later, with SQL Developer release 1.5.
You must specify a third-party JDBC driver or install a driver using the Check for Updates feature before you can create a database connection to a third-party database of that associated type. (See the tabs for creating connections to third-party databases in the Create/Edit/Select Database Connection dialog box.)
As an alternative to using this preference, you can click Help, then Check for Updates to install the JTDS JDBC Driver for Microsoft SQL Server and the MySQL JDBE Driver as extensions.