To add content to a campaigner, first mark the segment you want to add content to by clicking on it, and then go to "Content" tab.
If you are not familiar with segments, see this video for a quick overview > https://help.nowdialogue.com/en/articles/6966083-segments-overview
On the right, you can see the content units menu, where you can add new content units to the Campaigner, or assign existing ones.
If you already have content units in your account that you want to add to your campaigner, just filter "All campaigns", and assign the relevant ones.
To create a new content unit, click on "Add new content"
Then you can add a link for the campaign to lead to, headline, if you want also subheadline, and image from your gallery.
If you want the image shown on mobile to be different from desktop, you can ass a separate image for mobile as well.
If you have several segments to your asset, make sure you repeat the process and assign content to each one of them.
Good luck!