Hpw to configure DNS Forwarder

I am trying to configure a DNS Name server to forward queries to a Forwarder.

However, when I opened the DNS console and access the Forwarders TAB, the ADD button is grayed out and can not add IP addresses of Forwarders.

I did check mark the Enable Forwarders box, but the ADD button is still grayed out

Can anyone tell me why the ADD button is grayed Out?