There is an easy method to pair the sender with the receiving device - have you tried that? If you have been mucking about with configuration up to now, you may need to reset the devices.
Go to TP-Link Support and download the couple of manuals (PDF), if you don't have the paper copy or the small CD with utilities and the manuals.
The more complicated method follows (see manual for some instructions - a bit cryptic and not clear unless you know what it means).
Using TpPLC utility -
If you look on the back of each of the units - sender and each extender - note the password like RYHE-XXXX- (etc) for each. You should be able to connect via a Ethernet cable to one of them, and one by one ADD a device and set the passwords, and if successful they will appear on the graphical display. If you have only the 2 devices this is quick and easy.