First off - welcome to TechIMO
As to your problem - it would be helpful to know what OS you are running.
Also, I'm assuming this is the one that plugs into your ethernet card - I'm confused over the part about plug-n-play recognizing the powerline adapter. I think the only thing that the OS would recognize is the ethernet card - it doesn't know what's plugged into the ethernet port.
FYI - I have a similar setup using the Netgear version of the powerline adapters & haven't had any problems (they work much better for me that the wireless stuff I tried). In my case, I'm running WIN 98SE.
You do have to make sure everything is powered up in the right order. Router first, then the powerline adapters, then boot the PC. It does sometimes take several minutes after booting for the network to be accessible. And if you lose power to the powerline adapter, you usually need to reboot again to get back onto the network.