Jett's Core Aura

Just double click the Core and use the Solidifier that way. (the mini window that shows that and Star Dusts)

When I double-click the core, it unequips it...
Trying to put the unequipped core in the scrolling window doesn't give the dusts or solidifier as a possible "scroll" to use on it.
(Fortunately I can re-equip the core, but that doesn't help with the link skill)

Double-clicking the skill shows the Equip window.

Double clicking the solidifier shows the scrolling interface, as CherryPie said.

Double clicking a Star Dust shows the skill's window finally (the hexagon thing). Note that it asks you whether you are sure you want to use the Star Dust, which I assume you don't, so make sure you say "no".
For me, the little "Enhance" window on the side of the skill interface didn't show the Solidifier (non-cash-shop, 7-day thing) until I moved it from the last slot in my inventory to another.

Good luck!