In this lesson, I address how to set your permissions in order to use Campaign Influence. I reference the Campaign Influence Implementation Guide that lists the different related lists you can add to various page layouts, as well as a specific Permission Set License you may need to be assigned in order to have the proper permissions. If you do not see Campaign Influence as an available related list from the Opportunity Page Layout, then you are missing this Permission Set Assignment. Once assigned, you will then see the related list. We assign the permission set and then adjust the Opportunity and Campaign Page Layouts to include their respective Campaign Influence Related Lists.
Campaign Influence Implementation Guide (PDF) - https://resources.docs.salesforce.com/latest/latest/en-us/sfdc/pdf/campaign_influence_implementation_guide.pdf