You can see which items are eligible in your dreambox, there's a text in blue YOOXDREAMBOX15 in the bottem of each item.
Code doesn't work for me at all, but it's just me, it happened with last two promos, so I had to contact CS, something is messed up in my account. CS are not super helpful though responding something like pelase contact us again if you have any concerns, that's all.
Yup - exactly. If an item is in your dreambox without the light blue text under it, it won't work.