Help with PlayNC Master Account

Hi guys,
This is my first post on the Guild Wars Guru forum and I am really hoping someone might be able to answer a question of mine. I logged into my Guild Wars account yesterday and decided to change my login password. On clicking 'change password', I was then directed to the PlayNC website and prompted to enter my PlayNC acount name and password. Now, I can't for the life of me recall in the two years since playing GW, ever setting up a PlayNC Master Account. Is there something that I missed along the way?
Many thanks

EDIT: Ok, so I have just learned from a previous post that all I need do is simply login to my existing Guild Wars Client, then proceed to visit the online store and enter my desired login details for a Master PlayNC account. However, after going to the ingame store, I'm not entirely sure where to key in my desired PlayNC account details. My apologies in advance if I have missed something simple here.

The key should be in the box.

When you entered the store for the first time it should have said that you need a PlayNC account to be able to use the store.

At least, that's how it worked when the store first opened.

There should be an option to "Create Account" on the page somewhere. Or on the PlayNC main page, at least.

Thats correct, there certainly is an option to create a new account. I then created a new PlayNC account but under the 'Game Accounts' tab it says 'None'. I also tried entering my GW gaming keys but get the response 'invalid serial code'.