You can use a free action whenever you want. So, if you can pull out your wand as a free action, then you can have it out when you're hit, and use the immediate reaction. However, the feat in question does not let you do this on other creatures' turns.
I believe you need to be wielding a magic weapon to use its power. I can't actually find a rules reference for this, but I'll be assuming as much for the rest of this answer.
So, given that assumption, you need to have the wand in your hand when you're hit, as the trigger for Shield is "You are hit by an attack". I believe you would have to get the wand out by the time the enemy attacked, perhaps when they move into an adjacent square. I'm not sure if you could pull it out between an attack being declared and the dice being rolled, or even after the dice being rolled. This isn't Magic: The Gathering, and these timing issues are not as explicitly defined as they could be.
As far as when pulling out the wand "resolves", note that this free action is not a triggered action, and therefore any rules about when triggered actions resolve do not apply.
The Rules Compendium describes Triggered Actions on page 195:
A triggered action is any action that can be taken only when a specific trigger occurs. A trigger is an action, an event, or an effect that allows the use of a triggered action.
Since you are allowed to pull out your wand without having to wait for any specific trigger, the action is not triggered.
Drawing the Wand
I specified earlier that this works if you can draw your wand as a free action. As silversociety pointed out, Eldritch Fusillade actually says that you can draw or stow a wand as a free action on your turn. I found the full text for the feat in a revised version of an article from Dragon Magazine 402 called Tools of Two Trades:
Benefit: You gain a +1 feat bonus to attack rolls that you make with a crossbow or a wand. This bonus increases to +2 at 11th level and +3 at 21st level.
You can also draw or stow a wand once per round as a free action on your turn, and you can load a crossbow as a free action.
Because it is not normally your turn when you're attacked, this doesn't work because you can't draw the wand as a free action. If it was your turn, then it would work. So you could do this in response to an attack of opportunity, but you would really just want to get the wand out before even moving.
András pointed out that the feat Quick Draw may allow this. Quick Draw, from the Player's Handbook, says:
Prerequisite: Dex 13
Benefit: You can draw a weapon (or an object stored in a belt pouch, bandolier, or similar container, such as a potion) as part of the same action used to attack with the weapon or use the object.
You also gain a +2 feat bonus to initiative checks.
If your wand counts as an "object stored in a...", and activating Shield is an example of "use the object", then maybe this could work. However, you're drawing it as part of using the action, the action is an immediate interrupt, and the interrupt won't be triggered because you don't currently have the wand equipped, so you would never have the opportunity to draw.
Yes, you need to have the wand in your hand when hit. Yes, if you can draw a wand as a free action then you can do this right before you are hit. No, Eldritch Fusillade does not allow you to draw your wand as a free action on other people's turns.