Elite Regiment unlock

Hi,
I have boxed game and on back of brochure it says I can unlock elite regiment units for game. There is given key. I triad to look in steam and in game but I couldn't find where actually I should enter key digits.

Thread moved from NTW GD forums to NTW tech help.

If you have purchased the Napoleon: Total War™ Standard/ Imperial Edition or received an exclusive pre-order unit via participating retail outlets, you will need to connect to the internet, redeem your code through Steamworks and easily unlock your units for use in game.

1. Install Napoleon and the Steam client as per instructions included with the game.
2. Launch the Steam client and select "Activate a product on Steam..." under the "Games" menu.
3. Step through the Steam Product activation wizard, entering the product code as prompted.
4. The units will then become available in Napoleon's single-player campaign/ battle modes and multiplayer under certain conditions - see 'Where do these units appear in the game?' section.

^Source