If you forget to add a post to an existing campaign when you're creating it, or need to retroactively add a post to a campaign, you can do so in Post Manager. First, open Post Manager at the bottom left-hand corner of the Dashboard.
Using the Post filter, you can search for posts by keyword, URL/UTM codes, metatags, or whether not it contains a Branded Content sponsor.
When you find your post, click on it to open and then select the desired campaign from the drop-down menu. Close the post and you're all done - the post should populate in your campaign's stream and in your reports going forward.