There are several instances where we're asked to apply open credits to invoices on accounts.   Here are the steps needed to accomplish that goal.

1.  Find the right account in Store Manager under Customer, as shown here:

2. Open the customer account and click Ledger.

3. Locate and select the line item with the open credit and click apply.  

4.  On the next screen, make sure the amount and other info is correct, and click Next.

5.  On the next screen find and select the invoice that you want to apply the credit towards and click Finish.

That's all folks!  Let me know if you have any issues with these steps or any recommendations on how to improve them.