how can I get an activation code for netflix?

There is no code, you simply login to your account. You can go to the Netflix website and/or call their customer service if you are having issues logging in.

I don't currently have access to an AppleTV, so I can't personally test this. Keep in mind that Netflix streaming requires you to have an active paid account.

If the app on AppleTV works similar to the one for iOS (iPhone/iPad) then there should be a field to enter your Netflix email and password.

Otherwise for devices like DVD players and smart TVs (from the Netflix site):

Q:

How do I activate a Netflix-ready device?

A:

To activate and manage your Netflix-ready device, both your device and your personal computer need to be simultaneously connected to the Internet -- your Internet connection must be equipped with a router to enable this.***

To activate your Netflix ready-device:

• Follow the manufacturer's instructions to connect the device to your TV and the Internet.
• Navigate to the Netflix option on your device and select it.
• Follow the on-screen instructions until you see a screen that displays your Activation Code. Write down that code.
• At your computer, go to Netflix.com/activate and enter the code.
• Your device will be associated with your Account, and the instant Queue that you create on the computer will be displayed on your TV via your Netflix-ready device.

*** Broadband Internet access services typically provide high-speed modems with built-in routers, so you can connect multiple computers to the Internet. If you do not have a router or are unfamiliar with this technology, your Internet service provider should be able to provide you with more information.

If a router is not an option for you, Netflix customer service can assist you in activating your device, but you will still need access to a Web-connected computer to manage your Instant Queue.