No worries mate. You use the link I posted:
http://www.microsoft.com/uk/education/studentoffer/
then look for this section:
Windows 7 - What do I need to do?
1 - First, make sure your PC can run Windows 7 with the Windows Upgrade Advisor.
2 - Go here and validate your email address.
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You click on the word "here" and then you'll be prompted to enter your student mail address. They then mail you a link to the shop, where you can buy it. They're just trying to lock it down and make sure only students use the offer, by making it a "hidden" link which they only provide to the student mail address.
R.