Question: how to install jtds driver in sql developer?
Answer: Download the latest "jTDS - SQL Server and Sybase JDBC driver" from here.
Unzip the driver, giving you a "jtds-1.3. ... 
Open SQL Developer and navigate to "Tools > Preferences > Database > Third Party JDBC Driver".
Click the "Add Entry..." button and highlight the "jtds-1.3.