You have two ways to activate the new Notifications and Subscriptions module. It depends on if you are creating a new KB or a pre-existing one.

It should be noted that notifications and subscriptions are independent of the discussion module, but if both modules are imported, both are enhanced, since you can be notified in a discussion that you are mentioned or subscribed.

In the case that it is a pre-existing KB, to enable the new modules it is necessary to run the Design System Wizard.

In the first step select 'Current Design System' to tell the Wizard that you want to modify the current Design System. Then activate it in Step 5 'Include notifications and subscriptions module'. To finish go to Step 10 and press the 'Import Design System' button. 

Do not worry, with the new DSW, it will be imported only needed to run these modules thanks to the intelligence that has the new DSW as explained in Improvements in Design System Wizard.

• Another way to activate the module is when you run the Wizard for the first time.

In this case you must select in Step 2 'Include notifications and subscriptions module' and then 'Import Design System' if you want to run the wizard with the basic settings.
Also, you can activate it in Step 5 if you decide to enter 'Advanced Settings' to make other modifications and then go to Step 10 to import the changes into the Design System.