THis for those that don't want to have to Dl the 5.7gb from steam when they have the DVD retail version.

Using Steam launch options to install retail games from disc    Log in to Steam and click on Library.    Right-click on the game, select Delete local content, and confirm.    Insert the first disc into your computer.    Close Steam (Steam > Exit).    Press Windows Key + R to open Run    In the Run window type:    "C:\Program Files\Steam\Steam.exe" -install E:    Replace E: with the CD/DVD drive you are installing from if is not correct.    Replace C:\Program Files\Steam if your Steam installation is not in the default location.    Press OK.  Steam will launch and ask you to sign in if you do not have your password saved.  Your installation should continue from the disc. 

for MAC users check the link

https://support.steampowered.com/kb_article.php?ref=5357-FSQM-0382

Can some one make this a sticky so we don't end up with a zillion posts about this.

I tried everything because my Steam file is located in C:\Program files (x86) But it keeps saying that they can't find it...