It is not the location of the jTDS jar that is important, but rather registering it with a specific SQL Developer installation via
Tools > Preferences > Database > Third Party JDBC Drivers as noted in our documentation and several blogs, one of which is...
Also, since the 4.x and higher releases of SQL Developer require Java 7 or higher, you need a jTDS 1.3.x driver version, not a 1.2.x.
For some reason, I was pointing to the location of the jtds-1.3.1 jar file, not to the jar file.
I'm good-to-go now.
Thank you, Gary!