I specified an amount that was too low for my testing needs, when creating a personal paypal sandbox account. Is there any way to add money to an account, after it has been created?
If you need actual funds in that account you could use Send Money from another sandbox account and pay the account you want to fund.
You really don't need funds in the account to test payments, though. As long as the account is verified and confirmed it'll fall to "Instant Transfer" and you won't have any problems sending payments using that account.