So, natural search is kind of a tricky metric because it hasn't been introduced as a usable rule yet (despite it being used in a default segment). I've heard rumors that it's going to be introduced in May's maintenance release, but in case it's not, you can try this:
- Go to Star Menu | Admin Tools | Report Suites
- Select the report suite in question, go to Edit Settings | Marketing Channels | Marketing Channel processing rules
- Set them up however you'd like, just make sure one of the rules is 'Natural Search' (it should be a default rule)
- Once saved, you can go into the segment manager and select 'Last Touch Channel' equals 'Natural Search'
It results in pretty much identical data to natural search rule by itself, just with this method you're going through processing rules to make it part of the segment. Also note that this is not retroactive if you haven't set up marketing channels - if you already have marketing channels set up, it goes back as far as when you set them up.