A notification appears on the toolbar when new mystery items have been added. Click on the notification to go to your Inventory page (direct link). In the "Special" section, you will see a gift box, with a number that indicates how many items it contains. Click on it until the number reaches zero. You will then find the items in the "Mystery" section of your Equipment page (direct link). If you don't see them, reload the site and click on the gift box again.