All purchases from the Steam Store can only be credited to the Steam account linked to the game. To receive them, you have to log into the game using the Steam authorization button (not web authorization) on the account login screen (where you enter your username and password).
Please note that the Steam authorization button is only available in the old game clients. All new game clients automatically log into the game using the Steam account linked to it.
If the purchase was credited to the wrong game account, please contact support: How to submit a support request