You can only attach one product to your campaign. If you want to have a campaign that includes more than one product, you can create a product profile that includes more than one product.
For example if you sell Product A, Product B, and Product C and you created product profiles for each of those an now you want to create a campaign that includes all three, go back to Products and create a fourth product profile that has details for all three.
Then go back to campaigns and attach the fourth product that includes all three products.