Leo says that the domain name server (DNS) is basically the phone book for the internet. When you type in an address, the DNS then takes that address and looks up the actual IP address of the website. It's numbers separated by periods. Tom can change his DNS on his computer quite easily. On the Mac, it's in the network settings of OS X.
To configure Mac OSX, Tom should go to System Preferences > Network, and select the connection he wants to configure. Then go to Advanced, and choose the DNS tab. Click the + sign at the bottom of the DNS Servers box, and enter the 3rd party DNS server settings. Then click OK, and Apply.
He'll have to change it for each interface. That's why Leo suggests doing it through the router instead. Leo recommends OpenDNS.com. It's faster and is more secure for blocking sites he doesn't want his kids to see.