unfortunately this is incorrect, the subpolicy was already removed in SMC 5.8. This will be fixed in future documentation. Starting from version 5.8 automatic rules take care of allowing the DHCP Relay traffic, so you don't need to do anything. There's an automatic rule insert point in the Firewall Template, and if you have DHCP Relay enabled either in interface properties or in VPN Client settings, then rules for DHCP relay are created for you automatically there.
The automatic rules always have an Allow action, so only exception to above is scenario where you want the firewall to put the relayed DHCP packets into a VPN. In that case you need to add the VPN rules to the template above the automatic rule insert point. For this scenario see https://kc.mcafee.com/corporate/index?page=content&id=KB82134