To use your Reward points online then use the same email address in the checkout as the Rewards are allocated too. 

At step 3 of 4 "Review and Amend Order" (see below) you will see the Customer Rewards section and a button to redeem (your balance is shown here too). 

If you are not buying just yet then logging in to your account online and going to My Account also shows you Reward balance too.

More info on Rewards here.