Ad not appearing… "CSS file is not active here!"

I’ve been trying to place an advert and, despite the website saying that the ad is active, it doesn’t appear anywhere on the site.

Under Preview, it says, “Your themes CSS file is not active here!”

Under Usage, it says, “Copy the short code in a post or page. The PHP code goes i a theme file where you want the advert to show up.
In the theme CSS page, I tried copying and pasting the code and it disappears when I hit publish and nothing comes up on the website.

I use AdRotate.

Please help!