Even if it is an auto attack..
WRONG.
It is not an auto attack, an auto hit, an auto anything. It is a ranged attack power that has no attack roll and therefore does not hit anything. It has an effect that includes damage. That does not make it automatically do anything it does not do. On top of this, THIS IS NOT RELEVANT.
you have to see what you're attacking to attack it.
WRONG.
Go to your Player's Handbook, and open up to page 281.
You will find a sectioned called... wait for it...
ATTACKING WHAT YOU CANNOT SEE
"If you’re fighting a creature you can’t see—when a creature is invisible, you’re blinded, or you’re fighting in darkness you can’t see through—you have to target a square rather than the creature. You also have to figure out which square to attack. Here’s how it works."--It goes on into greater detail but you get the picture.
When you have a section of the rules explicitly devoted to attacking things you cannot see, and they tell you can, in fact, do so, then any argument that you cannot attack things you cannot see is automatically wrong.
This is one of the few cases of an automatic miss in the game.
Not to mention, you are aware of the location of any creature that is not hidden from you.
"If an invisible creature is not hidden from you, you can hear it or sense some other sign of its presence and therefore know what space it occupies, although you still can’t see it."
This is from the errata'd version, as well as from the PHB2. Page 224.
Immobilize would like to have a word with you as well then.
If you're immobilized you don't have the ability to do basic attacks..
WRONG.
IMMOBILIZED
You can’t move from your space, although you can teleport and can be forced to move by a pull, a push, or a slide.
Page 277, PHB. It says nothing about affecting attacks, ranged, basic, or otherwise.
I don't understand why that's so hard to understand.
IRONIC