Do not spray anything on your dress, what if it stained it or left spots. You would be devastated. Call Davids Bridal and ask them if you can bring it in to get steamed.
A professional steamer is a wonderful thing to have. Do not use anything other than a real steamer. Walmart has them in their small appliance areas next to the irons. Do not try to iron it either, it will melt the material of your dress. When an iron steams it is because the surface of the iron is hotter than boiling, it makes the steam when it touches it. Even holding the iron sort of close and hitting the steam button will expose the fabric to the extreme heat and it will probably melt the fabric of your dress. In a steamer the heat source is inside the hose and there is not heat plate or heat source that can come in contact with the fabric.
Practice on similar fabric. You can buy a yard or less at a store like Hobby Lobby then practice.
Each person will find a particular way they like to steam fabric. I find I like holding the steamer under the fabric sliding it down, the steam vents facing me. My friend likes to steam from the top of the fabric. She tears the flap/top of a box off and places that under the fabric and uses it much like an ironing board. Steam burns quickly, it makes huge blisters due to IT"S HOT, it can even leave scars if it is not treated quickly by a medication like Silvadene. It takes practice a few times and you'll have the hang of it. Then you can whip out the steamer and steam your clothes easily.
One thing you might try, hand the dress in the bathroom away from the shower but close enough to feel the steam, fully opened up and hanging down long. Take a hot steamy shower or just turn on the shower and close the door, sometimes I have to stuff a towel under the door to keep the cooler temp form the hall out. Don't let the dress get wet but the steam should penetrate the fibers and release the wrinkles. Try it today so that if it doesn't work you can get it steamed by the store.
So:
1st choice: Store steams it again for free or minimal charge
2nd choice: hand in the bathroom while hot water is making steam.
3rd choice: Buy a real clothing steamer from a store like walmart and try it out on a similar fabric to learn how to use it. It is easy, can be used at the location of your wedding to touch up the wrinkles from transporting it to the venue. It can even be used on the skirt if it's held away from your skin while you have the dress on....my maid of honor did that to my dress at my second wedding. I had sat in my dress for about half an hour without thinking about the train being hooked up. My wedding started about 15 minutes late but I looked pristine.
Congrats on the marriage.