Maintenance for the week of January 12:
• PC/Mac: No hay mantenimiento – 12 de enero
• ESO Store and Account System for maintenance – January 13, 9:00AM ET (14:00 UTC) - 12:00PM ET (17:00 UTC)
If you have a star that says it's slottable and you have at least the first tier filled in, clicking on in and dragging it over to the slots should highlight the appropriate section and you can plop it into one of the slots available.
It seems a bit finicky and they could make the highlighting a bit better, though.