I'm also very confusing with how to make ads work properly. Look like there is not any straight forward procedure to enable it.
I emailed partnersucces and they provide me a "roku_test_tag" which contains an ads URL. Passing that ads URL to setAdUrl API, I can see a pre-roll ads appear in my channel. But I have no idea how Roku can know that ads is playing from my channel to manage revenue (I look at the URL and don't see any kind of id that associated to my channel). I contacted partnersuccess again but didn't get response.
Another problem is that I always see the same ads after several days test, user will get mad if they see the same ads every time.