You've got a really old copy of SQL Developer, so not sure it's compatible with the version of JDev you're working with in terms of extensions.
If you grab version 4.2 of SQLDev, you can take a look at the Extension How-To's on our GitHub project
This is where we're burning calorie in terms of helping folks with extensions going forward, so I'd start here.
Thank you very much !
I've just tried with a newer SQL Developer and it works.
And thanks for the GitHub link, I'm sure it will help