Yes, you have to add a capsule of Amazon ads if you want them there. You have two options, you can choose specific items, or you can let Amazon choose what to add based on keywords you give it.
Specific ones make sure that your item is related to your hub, and people are more likely to buy it if it is related. But you have to keep checking your hub from time to time to make sure that the item is still being sold.
Keyword ones will refresh automatically but may wind up not having much to do with the topic of your hub, may not be bought as much, and sometimes I find that the same product sometimes appears more than once depending on how it was set up on their system.