To install the plugin in Unity: first, open your Project or a copy of your Project and the Scene you want to place an ad into in the Editor. Make sure this scene is saved.
Assets → Import Assets → Custom Package:
Browse to the destination of the downloaded plugin and import it.
The import process should take several minutes and create an "Admix" menu entry in Unity.
The plugin is now installed in Unity. Open
Admix → Open Admix to get started.
On the Profile page, log in to your Admix account with the same credentials you used on the developer platform.
You should see a confirmation that you are logged in.
Now you can choose an app name and click "Create" to make this project appear on the Admix Developer Platform.
Or if you have an existing project with Admix you can pick it from the list by choosing the 'Select' button:
Once the project is created and tied with the plugin you’ll be switched to the 'Create' tab where you'll be able to create your placements!