Just click on that Destrucitble (Or press enter while you have it selected) and it will have a Tab called "Items Dropped" and select "Use custom item Table" then just select "Add new set" after that just select "add item" and add what ever, if you want to make all objects drop the same (Or random) items, Go under the advanced tab in the world editor and select "Item Tables...", after that select "Add Tables" enter the name, then select "New Set" name it, then select "New Item" after that just put what ever items you want... I hope this was very useful.