SECONDARY MUTATION: When this power appears, stop turning the dial. You may choose to click Emma to click #8. Emma Frost can use Super Senses.

... considering the phrasing here, am I obligated to make the choice to move to click 8 only when I've stopped turning the dial, or is that option available to me as long as that power is showing? If the latter... when am I permitted to click her? On my turn? Anytime I feel like it?

From the PG:

014 Emma Frost Secondary Mutation reads: "When this power appears, stop turning the dial, and you may choose to click Emma Frost to click #8. Emma Frost can use Super Senses."

The errata now directly links the choice to the appearance of the power. You either choose to click when it appears or you don't click at all.