I tried to put the following into the Optional Parameters box -DSTART=extra/etc/start-plus.config -jar but all I got were errors.
You did the right thing, but you didn’t adjust your paths, they are still relative and considering MyEclipse launches JP from a different directory, they don’t make sense. Try this:
java -DSTART=<absolute dir here>/extra/etc/start-plus.config -jar <absolute dir here>/start.jar <absolute dir here>/etc/jetty.xml
Replacing the “<absolute dir here>” with the full path to that file.