If you have bought TE2013 from Mana Games, you can request your free Steam Key for TE2013 here : http://www.managames.com/Key_en.php .
To get your Steam Key, you have to sign in through Steam 1st, and then enter your Customer ID, and your email if it changed since you ordered the game.
If you have lost your Customer ID, only enter your email address (if it didn't change since you ordered the game) to receive your License Key email with your Customer ID in it.
Once done correctly, the Steam Key will be written in middle of your License Key email. You can then enter it in Steam.