You received a voucher code - nice! But how and where can it be redeemed? This is how it works:
Open your app and chose "My Account" in your account menu.
Then you chose "vouchers" in the list on top. Choose the blue plus-symbol and a window for inserting the voucher code will appear.
If your code was entered correctly, a preview-text with green background will appear - just click on "redeem" as a final step. You MILEScredits should arrive in your account at any second now.
Nice to know: Some vouchers can only be redeemed in the registration process and will not be available after your account has been created.
Have a great day and nice rides!