If you ordered from SE Store, you won't get your code until the 23rd America time.

If you ordered from Steam you have it now.

If you ordered from GMG, you should get it today or tomorrow.

As far as entering it, you have to click on the first menu option at the top (Your Account). Then there will be a big pink button that says, "Enter the Registration Code for the Expansion". Then you're done once you input it and follow the menus.