Well, i installed the game, installed the DLC pack and when in game (after intro/tutorial), in the DLC menu, it says "ALL DLC ENABLED" and i can choose the extra three campaigns.

Ok about the campaigns. But how do i get the ITEMS of those DLC packs? They are not automatically appearing in Death's inventory, like other games' dlc items...

You'll get access to them once you reach the first hub, they'll be waiting in the mail. Also note that they're level 5 items (and good until around level 10 or so), so you will most likely not be able to use them straight away.

You should also note that your inventory is limited to one page for each type of item.