No you misunderstand, I managed to find the big blue button, despite it being on neither of the pages the links in the email take you too.

The bit I am having difficulty with is this, taken from email :-

EXISTING CUSTOMERS: You can book online by simply logging in to your MySky using your SkyID. You will then need to check broadband availability. Once this is confirmed please add Sky Broadband Unlimited to your beasket. Below the basket you will see a "Got an offer code link" - please click on this and enter your unique offer code from above so that 9 months free Sky Broadband Unlimited is applied to your account.

The only way I can see the link below the basket is to be logged out.

When submitting the code it just goes to a blank page.