how to install the plugin?

I've never understood why Moodle doesn't just have this as a default option

I've installed the plugin in a Moodle 2.3.1. following the instructions. All seem to be right, but I founded two problems:
1. When discuss subscription is enabled "disabled" is not disponible, only "Enable".
2. When I am into a discussion and click the link "Subscribe to this discussion" I get back the error message: "Discussion subscription disabled for this forum".
What seems to be is not be really enabled plugin, but when I click again in Enable link I receive the ok: "Discussion subscription enabled for this forum". Please, ¿anyone can help me?

Hello sir,
I am new moodle. Could please help to create custom plugin to generate report

Thanks & Regards,
AS

Hello, it is a great plugin i have ever seen.
thanks for the sharing this. i also develop plugin about games at school

hello sir,
please help me out to install this plugin.
i want this on my 24h fun games site.
so help me to install this great plugin.
thanks in advance.

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Am I right in thinking this plugin is essentially part of the core now? Meaning there's no point installing it on later versions?