Hi Terry-
Welcome aboard the Banana Express.
As Max indicated, you need something coarse in there to aid with drainage. One of the favored items on here, I think, is granulated pumice. You can find it at any tack store or farmer's supply. It's called Dri-Stall or Stall-Dri (I think there are a couple makers), but they'll know what you mean if you use one of those names. It works great. I now use it in ALL of my soli mixes to ensure excellent drainage.
Regarding storing it by a sunny window: Keep in mind that Low-E windows and the like are designed to remove the harmful UV rays that plants need, so just because it's a SUNNY window doesn't mean it's a HEALTHY window.
Now, if your variety is Musa basjoo, you won't have to bring it in. Just plant the corm a foot or so down and it will survive your winters fine. Do you know what you bought?