Due to the crazy backend setup of RO2 and RS in Steam, if you purchase Rising Storm first, there is now way to activate Red Orchestra 2 Single Player on your account.
If you do want this content, you will need to create a new Steam account during the promotion and activate the game on that account.
3 Community found Work Arounds:
I previously bought rising storm through the humble bundle, and did not have single player access. I was able to add single player to my account through this method:
1. Log out of/exit steam entirely (Log out through your browser as well.)
2. Go to the red orchestra store page:http://store.steampowered.com/app/35450/
3. Click on install game
4. Steam will ask you to log in through your browser.
5. After you are taken to the "success" page, click on the "Yes, I have steam box"
For the URL error try these steps:
Go to http://store.steampowered.com/app/35450/
Login (URL error here)
Now choose to go back 2 pages in your browser so you are back to the app/35450 page without reloading the page(worked in Firefox). You should be able to click the Install Game button and add it to your account without logging in.
Log out of the Steam client and close it.
Log out of Steam in your browser and close it (I also cleared my recent history/cache)
Reopen your browser and go to the Red Orchestra 2 page - http://store.steampowered.com/app/35450/
It should say Install Game, but DON'T click it yet.
Open a NEW browser tab/window and then log in to Steam.
Return to the Red Orchestra 2 page and now click Install Game. You should then receive a confirmation that it has been added to your account, and it will be reflected when you restart the Steam client as well.