To use this WeakAura, equip then use (right-click) your trinket while not being near an Oathstone (for example, in Valdrakken) and it will activate.
When you are done, use the Close button at the top. Note that the weakaura is not active while in an instance.
Since this trinket forces you to go around the world to change your attunement, this WeakAura makes it a bit easier ! It displays a convenient list of all attunements with quick buttons to place TomTom or in-game markers.