En se référant à la FAQ du site :
I PURCHASED WITHOUT MAKING AN ACCOUNT AND I CAN'T LOGIN TO KSP STORE.
After you checkout from KSP Store, the system will automatically create a KSP Store Account with the email address and password you type in the Checkout Form. You should be able to log in using those details and the instructions here (https://kerbalspaceprogram.com/kspstore/instructi
ons.php).
If you do not receive a confirmation email, due to your email provider blocking it, don't worry, you can still log in and download the game.