First time with this module... it won´t show the popup... just a black space at the footer

Hi there!
This is my first time with lightbox, and it seems that I´m missing something very obvious here for this module to work.
I´ve installed it and followed all instructions. I want to embed a youtube video and when people clicks a link it should appear the black screen with the video in the middle of it.
I´ve not changed the default settings, except the one that applies to video, to enable it. Just that checkbox on.

Then I´ve tested the module: I´ve copied and paste the example code that comes within the readme file, but it won´t work. I´ve tried several variants, and this is what I get:
A normal link to youtube page.
or
A big black rectangle right after the page´s footer.

I´ve added the test code inside a tlp.

What am I missing here? I would really appreciate your help!
Thanks!
Rosamunda