I believe if you have Whole Home DVR active on your account you can order the Wireless CCK for $25 + $7 shipping right on their website. Or you can call them and try to get them to ship you one for free. I really would recommend this if you already have everything else in place to use Whole Home over DECA. Do all your receivers have it built in? DECA is built into the H24, H25, HR24, and HR34.
Otherwise if you really want to do it over ethernet then all you should have to do it hook up all of them to ethernet and then reset all of them so they switch over from DECA to using ethernet. You may also need to restart your router to get it to see them all and assign IP addresses to them. You may also have to reset network defaults on some of the receivers if they fail to switch over to a local IP address assigned by your router. Many people also prefer to set them up with static IP addresses outside your router's DHCP range. This is how I have Whole Home DVR set up in my house. The only reason I have it that way though, is because my gigabit ethernet network was already in place and I was already using it as part of the BETA test of Whole Home DVR before DECA even became available. And once DECA was available I didn't feel it was worth the extra cost to switch my system over because I would have needed 4 receiver DECAs and a CCK. I wasn't willing to pay DirecTV's cost for them to do the upgrade, and I'm still not. I have contemplated switching over myself someday when I have time to shop around and get all the parts for a price I am willing to pay. But, if all my receivers had DECA built in I would definitely switch over if only to simplify everything and increase speed/response on both my DECA network, and home ehternet network by seperating a lot of the traffic.
But I guess if I have to choose I'd prefer it over ethernet since I have ethernet in all the locations I want to use whole home anyway.
