The way for end-users to create a discussion thread in one record, or add a discussion message in an existent thread is by the Standard Action 'Discussions'. This Standard Action has to be in the View object of all the transactions that you want to have Discussions enabled.

This is done automatically by WorkWithPlus for Web when you run the Design System Wizard, as it adds the Standard Action in all the templates of View:

So, if you had WorkWithPlus for Web in your KB, and transactions with the Pattern applied, you need to make 'Build all' after running the Design System Wizard in order to visualize the 'Discussions' action in your view objects. It is important that if this is your scenario, you make WorkWithPlus > for Web > Pattern instances > 'Mark all instances as updated' prior to run the Design System Wizard to enable the Discussions Module, so that when WorkWithPlus for web makes the 'Update all instances' process (at the end of the import process of the Wizard), the only changes that apply is adding this Standard Action.