As I read it, it is a free action, not an interrupt (because, of course, it cannot be an interrupt). However, unlike Elven Accuracy, it does not specify that you can add it to the roll after the fact. So I think it is intended to be declared before the attack.
However, does anyone not think that Wand Mastery is the most useless of the implement abilities?
Given the relative weakness of this mastery (unless someone can convince me otherwise) I think that ruling that it can be used after is one step towards correcting this weakness (although not, imho, what they intended).
But I think that, compared to the others, even with that it is too weak and should be changed in the errata to always grant a +1 to attack, with the option of declaring that one attack per encounter has a scaled bonus to hit. This makes it directly comparable to the staff which always grants a +1 to AC and an ability-scored scaled bonus to defense (immediate interrupt, after the attack) once per encounter.
Carl