The key in the Customization Manager to specify where the data source is located is BillingManagerPrintDataPath. Be sure that the BillingManagerPrintDataPath ends with a backslash. For example, C:\Program Files (x86)\ILLiad\Print would not work, but C:\Program Files (x86)\ILLiad\Print\ would.
More information can be found in the Printing Billing Invoices documentation.