|If you mean to say the management interface on your (router/range extender) isn't opening then you need to reread the manual and ensure you're following the steps correctly.|
A couple things to remember:
If it's your router you're trying to access you can open a command prompt window and do a: ipconfig /all command. The Default Gateway IP is the one you need to connect to in your web browser.
If it's your range extender you're trying to connect to the computer you're using to connect to it has to have an IP in the same subnet as the extender. If you normally connect said computer to your router it's likely the network interface on your computer is set to get it's TCP/IP settings automatically from DHCP. You would need to manually assign an IP within the same subnet as the default IP of the extender.
Range Extender's default
IP address = 192.168.0.50
Subnet Mask = 255.255.255.0
IP for computer NIC should be:
Address = 192.168.0.10
Unplug the PC from your router, plug it into the network port on the range extender, open your web browser and type the extender's IP (192.168.0.50) into the URL field to connect.
It's also worth mentioning here to check the DHCP scope on your router and it's LAN IP before configuring the extender. You'll want to give the extender an IP in the same subnet but outside of the DHCP Scope.
It matters not how straight the gate,
How charged with punishments the scroll,
I am the master of my fate;
I am the captain of my soul.