The details on Fogs website are only for Cisco IOS and not Cisco PIX OS. I think its important to point out here that whilst some similarities exist between the 2 operating systems, for instance show run can be done in both, they are totally different.
This is why you cannot find the “ip dhcp-pool” statement in your configuration. The commands concerned with DHCP in PIX OS all start with “dhcpd” but as far as I can see there is no way of configuring the next-server option you require to specify the Fog server as the next server in the boot process. There simply is not the command.