Adding creatives to your campaign is super simple!
First off, choose the campaign you want to add creatives to, and click on it.
Now, you should be on the Campaign Report page, and you'll see the button Add Creative in the top action bar, like so:
Click on it, and you'll open up a new page, similar to the Campaign Flow, letting you add your Creative(s)!
After that, you'll just need to do what you would if you were creating a new campaign!