Can't access configuration page of TP-Link network expander (TP-AV500)
So I had the same issue but solved it both on the AV500 and AV600,the urls like this that TP-Link tell you to use http://tplinkplc.net rarely work because that url and even TP-Links software utilities are looking for a specific internal IP address for your extenders but your routers DHCP didn't assign that IP address.
You need to do is find the internal IP address for your extender and ideally set it to stick to a specific IP address.
Solution
After you have your Powerline connection sorted, connect into the extender/expander via an Ethernet cable.
Download the Powerline Scan Utility from TP-Link's website (The scan utility is all you need, don't worry about the proper utility).
Make sure you get the Scan Utility for your hardware version, you can see that on the back of the extender plug. You're router won't always show you the devices for some reason hence why this tool is mentioned.
Then when you launch that scan tool it will show you the internal IP address that you're device has been assigned and there will be a connect button, press that and login with admin/admin.
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Once inside go to the Network tool, change the IP address to something you want. Then copy the MAC address displayed into you're router and reserve that to the same IP address in the router.
Tips:
Change the admin password on the TP-Link extender, its admin/admin by default.
You can then also set it to mirror your routers SSID by changing the SSID name and password to that of your main router and then setting the security protocol to WPA-PSK/WPA2-PSK , then press the reboot link.
You can also enable a temporary guest-ssid in these settings if you have a new TP-Link device, then you can prevent connected devices from being able to access admin pages on your router and powerlines.
Actually the kits being sold don't have a cd. I was able to download the utility from TP-Link based on the model (WPA 4220 Extender version 1 in my case). Once I installed that utility on my pc it came up with the installed plc network and the wifi extenders had a "website" popout that brought up the browser admin admin configuration tool. At that point I could note the ip address for each in my browser. In my case they were http://192.168.0.225/ and http://192.168.0.9/ which then worked directly without the TP-Link utility.