Are you trying to configure the newly bought WiFi Range Extender using your Mac computer? If yes, then you need to open mywifiext local web address because the one with .net extension will not work on Mac. If mywifiext local is not working on your computer, then check if the internet connection is active. If it is, then take help of Extender experts.

Mywifiext.local login setup wizard

Netgear Extender Setup