Teaser break

The Teaser Break plugin is included in Wysiwyg API and the TinyMCE implementation has support for it so you do not need to edit any files.
All you should need to do is to activate the Teaser Break button under "Buttons and plugins" on admin/settings/wysiwyg/profile/#/edit where # is your profile number.

Enabling this option gives me a teaser button in the toolbar, but it seems to be disabled.

If it's there it should be working. Clicking it should insert a narrow image with a red text saying <!--break--> which is a placeholder for Drupal's <!--break--> comment as it would otherwise not be visible in WYSIWYG mode. In source mode, or if you disable Rich text editing, you should just see the comment as that's what will actually be stored in the content.

If it's not working, please look for JavaScript errors and report them here.

Well, the problem is that there is any "teaser break" button or plugin to activate in my admin/settings/wysiwyg/profile/edit/2!!

Ok, the list of editor plugins is hard coded into the editor implementation so if the editor is found, the plugins should always appear. But let's start at the top just to be sure the very basics are correct.
Please confirm that:

1. Wysiwyg module 6.x-2.0 is installed and enabled, and TinyMCE 3.2.2.3 was extracted to /sites/all/libraries/tinymce?
2. TinyMCE 3.2.2.3 is recognised as being installed on admin/settings/wysiwyg/profile and is selected for Full HTML (filter/profile 2)?
3. admin/settings/wysiwyg/profile/2/edit shows a three column list of buttons and plugins which can be enabled for TinyMCE 3.2.3?
4. At the bottom you should see an item named "Teaser Break".
5. Enabling only Teaser Break plugin should show a single white icon in TinyMCE's toolbar.

unfortunately i'm not at home. I'll reply asap

I can confirm this may be a real issue. I have been seeing on a couple of sites the teaser break button is activated but the icon is greyed out. I will look into this more as it may be a permissions issue but there is no explicit permission for the buttons on the module.

@Shane Birley, see #497654: Drupal plugin buttons disabled in WebKit based browsers for why you get grayed out icons. This issue is about it not being able to enable Teaser Break at all because it's not listed as a plugin.

Thanks for taking the time to report this issue.

However, marking as duplicate of #497654: Drupal plugin buttons disabled in WebKit based browsers.
You can follow up on that issue to track its status instead. If any information from this issue is missing in the other issue, please make sure you provide it over there.