Don’t worry. We like to think our selves as people of the world .
Maybe some of the information is out of date because we have been renovating the services, but let me explain it to you.
You will need to create and external service with that code : http://dashboard-dev.cristiantala.cl/index.php?r=services-bridge-usuarios%2Findex
After adding the new code, as an external WebpayPlus PST Service you can assign it under your own service. Just select your service in the following link : http://dashboard-dev.cristiantala.cl/index.php?r=servicios-tbk
On the button list you will see one that says add a external service, on which you will add your just created external service.
That is how you assign the commerce code to your service. We are doing this because soon you will be able to use several payment services, so the process should be equal to all of them.
Talking about the callback url, it is only shown when you select a custom kind of service, that way you can access Return URL and Callback URL variables.
Let us know if this work for you. Wish you the best !