Within the admin console, navigate to your web app via Web Apps > [select your web app], then click the 'Edit Web App Settings'. Go to Layout > Detail Layout.
From the toolbox on the right, click 'Modules' and you will be able to insert the comments submission form module and the module tag to render comments. Additional information on comments can be found here: http://docs.businesscatalyst.com/user-manual#!/crm/comments
