Stickying this info would help new or rejoining members of the community.

It is highly disappointing that this info is not made clearer by Steam, Gamestop, or Creative Assembly. You can enter the same Fall of the Samurai link you already use and the content will now be available under 'Start new Campaign,' and any other menu option where the normal Shogun 2 game options would be found. If Fall of the Samurai and all related Steam update files have already downloaded and installed, no additional files should download. After a few moments, the game should activate within Steam. Select 'Activate a Product on Steam.' Enter the product key (including dashes) into the field provided, and click activate-be aware that Shogun uses a rather long string, so make sure you get all of it correct! Click Activate.

Go to Steam and click on the Games drop down menu option from the list at the top of the screen. To access the content, you must find the Shogun 2 product key (for those of you with gamestop's Impulse service, install the service, redeem your code, and right-click on the title in Impulse and select Product Code). However, the content of the Shogun 2 core game is not accessible. If you install Fall of the Samurai, Shogun 2 is installed as well. 'This issue is plaguing players of Fall of the Samurai who did not first acquire and install Shogun 2. Copy the Game Key and paste it in the Product Code field, then click. Select ‘I Agree’ option, since Steam Subscriber Agreement needs to be agreed with in order to continue. After the Product Activation window pops up, select ‘Next’. It is indeed confusing and a shame steam doesn't explain it clearer. You will get 3 options from which you need to select the ‘Activate a Product on Steam’ option.

Shogun 2 steam product key