The description says if you are not attuned you must make a DC15 wisdom save or take psychic damage. But the character sheet interface does not allow a player to choose it as an attuned item.

In the inventory, click on the Mandolin and expand it. There is an "Attune" box at the bottom.

Sorry I forgot you had to ready it first. I was looking at it from the limited use pane instead of the equipment pane. 

Thanks!

Does this item recharge at dawn? Doesn’t specifically say and long rest doesn’t recharge it…

It is per dawn.