Each Campaign is linked to a Campaign Product. Multiple campaign products can be setup per instance, however only one product can be linked to a campaign.
Create a Campaign Product
Navigate to Menu item CRM Contacts | Leads | Campaign Products
Click on the + New Product button to create a new campaign product
Complete the required fields
NOTE: The button name and link required on the Campaign Product screen is displayed on the lead view and provides quick access to the product information of marketing brochure to the agent.