My problem is that under the Credits tab, I can't Add Credit To Invoice or Remove Credit From Invoice because both boxes say that $0.00 credit is available. I had assumed that adding a credit under Manage Credits would make funds available to apply to the invoice under that tab.
Under the Client Summary, it says Available Credit Balance: $0.00. Yet the credit I entered under manage Credits is still there.
ANSWER: Apparently I needed to also add the credit amount to the Profile tab under Client Profile to make it work. Now it works as I expected but it wasn't clear to me that the Manage Credits entry was purely informational - I assume - as a backup record for my manual entry of the amount under the profile tab.