How to add programs to nurture streams?

Here's the community article on how to do it: https://community.marketo.com/MarketoArticle?id=kA050000000LAW4CAO#add-a-program-to-a-stream

The main difference is that with the program, you have to select a smart batch campaign within that program. When the cast runs, it runs that batch campaign, which generally has a "Send Email" action. You would be able to add your filters to this smart campaign to prevent people who have already received this content from getting it again or add constraints to send them alternative content.

Thanks - So do you activate the campaign within the program? This is the part I am confused on. I have a program built that includes a campaign to filter out those who already downloaded the content and then includes a send step... but it doesn't seem right to choose a date/time for the send when it's happening within the nurture program. 

Right. It's not a triggered campaign, so you don't activate it, but you also don't set a date/time for it to fire. By setting it as the smart campaign in the stream, the cast knows to run that smart campaign when the program is next in line.

When a program is part of an engagement program, you can completely ignore the "Schedule" tab. The EP cadence overrides all of this. As long as you include the appropriate smart campaign (which also must include a "member of engagement program" filter) that defines the email to send, you're all set. Also, the other way that someone can skip receiving a specific email is if they are already a member of the program. If that's the case, the lead will receive the next email in the cast.