Once the air is 'pushed' into the attic, where does it go? (By the way, you're right - What you're looking for is a Whole House Fan. Go there to see how to install one ...)
With a Whole House Fan, you need to be sure you have Soffit Vents, Ridge Venting, and/or Gable Vents. You see, when the Fan ISN'T on, you need a Passive Cooling System. These vents properly installed allow Mother Nature to cool the attic when the Fan is not running. Do you have some sort of venting in the attic now???
Assuming the air from the house doesn't go ANYWHERE once it enters the attic, well, the Physics Law Of Equals comes into play. What this means is the following: Air Into Attic = Air Out Of Attic. Somehow, IF there's enough positive pressure generated by pushing the air into the attic, it's going to eventually have itself FORCED back into the living space (and some outside too.) Therefore, as for the air that ends up back in the living space, you are now HEATING that space because the attic air is very warm, thus you're having to Cool the living space now, and you're caught in a vicious 'circle'. This is why I'm asking about your attic venting.
In short, the air going into the attic MUST eventually be vented outside. As I mentioned, if your unit doesn't come w/a thermostat, you can install one in the line.
My best to ya and hope this helps. (If not, post back ...)