Your card spends from its own US dollar balance. Here's how to top it up:
On the Home screen, tap Top up.
Choose the balance you want to pay from.
Enter the amount. You'll see exactly what leaves your balance, the fee, and what lands on your card.
Confirm.
The money is converted to US dollars and arrives on your card within seconds. You won't need your passcode for this, because the money stays within your own account.