To add products/ASINs to your campaigns, take the follow steps:
1. Go to Ad groups tab and select the checkbox next to the ad group(s) you wish to ad products to. Then select the drop-down: Action > Edit Ad Group Settings. You will then be redirected to a new pop-up window.
2. Enter the individual ASINs of the products you wish to add within the "Select Products" field. Then click Apply changes.