Q: How do I redeem my netflix card through itunes?

To the best of my knowledge, you can't. Unless there's something I can't find, you can only redeem a Netflix gift card through Netflix' own web site:

http://support.netflix.com/en/node/2073

Regards.

I contacted Netflix and you can do it.  They have to cancel your account and open a new one but if you contact them they will give you the same rate.  Haven't tested it yet but I trust them.