Cloudyn or Azure Cost Management is a tool which helps to analyze Azure costs.
Usually activating it is pretty easy - just go to Azure portal, select "Cost Management + Billing", select Cloudyn and click "Go to Cloudyn"
If you are using Azure Enterprise subscription, then you need API Access Key which can be generated from ea.azure.com portal.
Once you have it, enter it during Cloudyn activation.
If you receive "The specified API key is not a top level enrollment key" during activation, then it means that the account which was used to generate API Access Key does not have full admin permissions in ea.azure.com portal.
If there is an Enrollment section in ea.azure.com portal, then it means you have full admin there.