As mentioned in the directionx for the AMAZING Lemon Cake from a Mix recipe
A flower nail looks just like a nail with a fairly large head, I am talking like half-dollar size.
People use them to make BC roses on, before they transfer them to a cake, hence the name flower nail.
In that particular instance, they want you to use it as a heating core in the middle of the cake. It cooks the cake more evenly, and the inside cooks as fast as the outside so that the outside doesnt get overly done.
Just grease your pan normally, and put the flower nail upside down in the center, and spray it well with cooking spray. Spread your cake batter like normal, covering the nail, and bake like the recipe tells you to.