Q. How do I activate Windows 10 clients using KMS?
A. To activate Windows 10 clients in your network using KMS you require an updated KMS key which will activate Windows Server 2012 R2 and Windows 10 clients. Once this is installed on your KMS server you will need to have the required number of machines to attempt activation before this new key will begin to activate (5 server OS instances to activate servers and 25 client and server to activate clients).
An update is required for a Windows Server 2012 and 2012 R2 KMS server to activate Windows 10 machines and this is deployed automatically via Microsoft Update or you can manually deploy https://support.microsoft.com/en-us/kb/3058168. Note at time of writing Windows Server 2008 R2 cannot activate Windows 10 but an update will be arriving shortly as documented at https://technet.microsoft.com/library/jj612867.aspx.