Double Nat?

[nj908]

Hello all. I have a question about routers.

I have an airport express (first generation) that I use for Wifi. However, I also have this Netgear router (N300). I bought it with the intentions of hardwiring my Xbox into the Netgear, simply because online gaming could get a little laggy when played wirelessly. However, other people in my house need to use the Wifi. I have tried to set up the Wifi from the Netgear router, and I was successful, but I can't password protect the Network for whatever reason. My question is, how can I set up the airport express into the Netgear router to use it for Wifi? The airport express keeps saying I have a "double nat" and basically yells at me and will not work . How can I fix this? Thank you in advance.