I do not know what would have caused the payment to be in "Denied" status, so you or the customer would need to contact PayPal Customer Service for any more detail on that.
However, if the customer just needs information on how to add funds in order to pay you, usually this is a simple process. The easiest way for them to pay you would be the following:
1. They log in to their PayPal account 2. They click the "Send Money" tab at the top of the page 3. They enter the email address of your account and the amount to be paid 4. On the next page, it will show where the funds are coming from - either the PayPal account balance, or a bank account, or a credit card, or from Buyer Credit. Even if the account balance is currently 0, the customer can usually pay you using one of these other methods (they do not need to "add funds" to the account first).
If there is any problem in this process, the customer will probably need to contact PayPal Customer Service directly to resolve the issue.