Hey, thanks for your interest! Always glad to field questions.
You spend 1 Insight or take 1 Arthic hold on 7-9 AND on a 10+ -- I wrote this as "When you roll a 7+" which might not have been the clearest. Basically: Anytime you get the effect of your Art, you pay a little bit of Insight/Arthic hold to represent the sort of "MP cost". Arthic Hold clears fast, but is a problem if you expect to be casting repeatedly in a single scene, while Insight recovers slowly, but there's no penalty for using it other than not having it.
On a 6- on a support Art, the GM should still make a move per page 53; What that move is will depend on the situation in which the art is being used, but you can usually fall back on Deplete Their Reserves (and make them spend Insight even without getting the effect), Inflict a Condition (Give them Drained, with or without getting the effect), or Ask Them What It Costs (and ask them what goes wrong.). Alteratively, you could use the opportunity to Foreshadow Something Ominous and just cut away to bad things happening elsewhere.
I probably should add information about this to the rules text though, since I think the support Arts are the only Moves that don't have an explicit 6- instruction.