Download the Platform Independent package, then extract it and locate the mysql-connector-j-….jar file.
I extract mine under C:\Program Files, but location is arbitrary.Open SQL Developer
Go to Tools → Preferences
Navigate to Database → Third Party JDBC Drivers
Click Add Entry… and select the mysql-connector-j-….jar you downloaded
Click OK
RESTART SQL Developer
Click New Connection (green “+”)
In the connection dialog, set Database Type to MySQL (in newer UIs this is a dropdown; older versions used a tab).
Fill in:
Hostname (or IP)
Port (usually 3306)
Database (schema name)
Username / Password
Click Test → then Connect