Please follow these steps:
- Copy Unity Ads link from the download menu in your Playground account
- Download the Android or iOS test app from Unity
- Open the app and past in the URL of your playable ad
- Press "Initialize"
- Once the ad is loads, press "Show"
If the ad fails to load, or to check the status of the ad, go back to the main screen and press "Analyze". Any issues will be surfaced in the Creative report
You can read more about this here.