A common pattern that Kubernetes implementations follow is to use Namespaces to indicate a team, app, or cost center that is responsible for a portion of a cluster. Often this is done to attach authorization policies to portions of a cluster, but can also be used to allocate costs along the namespace boundary.
When first landing on the Container Cost Allocation report, you will be looking at a view of all your clusters, and showing each clusters share of the total cost of all observed clusters.
Next, click on the "By Namespace" tab to Allocate by Namespace across all observed clusters.
To see Namespace Allocation within a given cluster, select that cluster in the clusters dropdown at the top.