Here is a step by step method to find your serial key
1) Hover the mouse over your name on the top, click on games.
2) Scroll down to the game Mount and Blade: Warband, Click on it so the screen come up below it.
3) Find the name of the game in the area below. Find right below to the right the More button with the down chevron., Click on it.
4) Find the option named Serial Keys, click on it.
5) Copy your serial key!