To search Post Planner for content from Twitter, go to FIND and click the search icon in the top navigation:
You can also go directly to Search via this link:
Once you're in Search, just type any keyword or handle and you will see a row of results for Twitter:
From there you can click on any Twitter profile and you will see the content for that feed:
From here, you can save the feed to one of your streams, or create a new stream to save it to.
Or you can just look through the content and see if there is something you'd like to share.