Ideally you would keep this in the same environment as you plan to use it in. Store it in the house somewhere, closet, cabinet, etc.
I would not store it in the attic. However if it is still sealed in the box and wrapped in plastic (?) you could store it in a shed, garage or basement. Without proper humidity you will run the risk of expansion and warp as bubbacuse77 stated.
This product being prefinished had the stain/finish baked on so this wood may be dryer than a raw wood floor installed then finished.
However the prefinished floor is moved around on truck, left in warehouses, and then moved to the sales floor. So as long as you have some controlled environment it should be fine.
If you ever need to replace a section of your floor let the wood sit in the house for a few days or couple of weeks to become acclimated to the change.