they have FAQs about this.
http://www.tp-link.com/en/article/?faqid=258
http://www.tp-link.com/en/article/?faqid=319

Dear Friends

I have the same problem,

Originally my home network are based on TL-PA411, I connect my PC, Satellite Receiver and the Router all works fine later on i added a TL-WPA281 AV200 Wireless N 300Mbps and it worked fine. BUT when i added TL-PA251 KIT to my network they don't communicate at all and using the utility software they don't see Each other at all. the problem i bought 3 Kits which is 6 Pisces in total !!?? i didn't expect this thing to happen. any way i have read the replies to this thread and followed the links and it didn't work. so please do you have any firmware to these kits to make the communication possible ?!

I bought this KITS to be used in my fathers Office for Cisco 7940 Phones hope i can find my solution here.

Waiting For Your Reply
Thanks

The link will help you.
http://forum.tp-link.com/showthread.php?75985-How-to-build-a-secured-powerline-network&region=china
PS:I think Method 2 is the best way for you.