The box to put code in is to the right of price.
I signed into U2.com,
Followed link fromresales page to tickle master
Selected date I wanted
On next page, was presented with 4 rows, experience presage, innocence presale followed by 2 more
Selected 2 tickets on 2nd row Innocence, price displayed to the right of this, and there was a box to the right of that, this is where you put the code.
Hope that helps