I have Oracle SQL Developer already installed and am able to connect to and query Oracle databases.
Using Help -> Check for Updates I was able to install the Oracle MySQ
Although @BrianHart 's answer is correct, if you are connecting from a remote host, you'll also need to allow remote hosts to connect to the MySQL/MariaDB database.
My article describes the full instructions to connect to a MySQL/MariaDB database in Oracle SQL Developer:
https://alvinbunk.wordpress.com/2017/06/29/using-oracle-sql-developer-to-connect-to-mysqlmariadb-databases/