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You'll have to select your "Bill to" account first.  This is the account that the purchases made will be billed to.  You can change this at any time by clicking "Change" in the top right navigation.  Keep in mind that doing so will clear your cart.
You'll then need to select your "Bill to" account. This account will be billed for the purchases made on here.  Again, this can be changed whenever you'd like by clicking "Change" in the top navigation.  This will cause your cart to be cleared.