How to unpack skins sets while in game lobby?

I know unpacked items can be equipped in the armory. That's not what this question is about, this is about items in a package that must be unpacked, then the items can be equipped individually or as a set.

I like to play random and I often have a skin available for that hero. But it's nearly impossible to unpack and equip that skin. I've managed to equip one while outside, but have no idea how any more. I keep clicking on the package and going between different menu's. I can't find an "unpack" option anywhere.

How do I unpack and equip skins that the game says are available?

Screenshots, I reduced the size to 25%:
Here I just randomed Tidehunter and it shows one item set as "in treasure".
So I click on it which brings me to this screen:
The array of heroes. I have no idea what this is or what's going on here. The whole thing is extremely confusing. But I can usually manage to find the hero that I'm looking for. So then I click on that heroe's name: The focus screen shows two item packs. One as "purchasing option", although I have no idea how I could get that, I don't want to spend money, so I don't care. And the package from before. I'm clicking all over the place but there's no unpack option. I just keep going back and forth between screens 2 and 3.
Afterwards I went to the inventory and now the skin pack is not available in the armory:
So wtf is going on here?
It's like they designed the interface to be obtuse on purpose.

The wording "Available - In treasure" means that the treasure is available for you to purchase, from which you have a chance of getting the item set. It doesn't mean you actually own the set or the treasure.

You can see the option to purchase the treasure in one of the screenshots in the question.

You can't purchase the set itself, you have to purchase the treasure, which may contain any individual set listed.