Unity Ads 1.x has now been removed from the Asset Store and the public release repositories.
Live games still using Unity Ads 1.x will continue serving ads and monetizing.
For new integrations please use Unity Ads 2.0
Integration guide for Unity: Unity developer integration guides
Integration for Android (native): Android developer integration guides
Integration for iOS (native): iOS developer integration guides
Unity Ads 2.0 Asset Store -package: Unity Asset Store - The Best Assets for Game Making
Note: Unity Ads 2.0 is available via the services window integration since Unity 5.5 - earlier editor versions still use version 1.5 - if you want to use an earlier editor with Ads 2.0 - please use the Asset Store package.
If you encounter any issues or have any questions - feel free to post them here or raise a support ticket by sending an e-mail to: unityads-support@unity3d.com