First, the new system rocks, you just have not figured out how to make it rock yet. That is not an insult, just want to be clear it will do whatever the hell you want if you figure it out. First pic shows the options when you are in your stream. Click on content types...topics and the second pic will pop up after you hit the little gear. Then click each of the forums you want in your stream and they will go dark as in the pic. The ones you don't pic won't be shown in your stream. After that is done click off the settings and you will see the third pic where you can save as a new stream or save as your current stream or discard.
Even though I am a genius it took me a couple of tries to get it right. So hang in there.