Well, it depends on the environment you want to use. Today we published a new plugin for Admob and Mopub. But right now it is just available for Webview+. In addition, you need to use the cordova exporter, because this plugin is still not included in the cloud.
In case you are interested, here is the repo: https://github.com/ludei/atomic-plugins-c2
If what you want is Canvas+, then the only option you have at the moment is to use Admob through Mopub.
Here is the plugin and a demo for C2: https://github.com/ludei/Construct-2-plugin
If after having a look at it, you still can't see any ads, you can visit our community and show our support team the device log. They will try to help you.