Assuming you have added the query-based DE (QBDE) to the BP in uDesigner, you do this in Unifier, under Company Workspace tab. Switch to Admin mode, then navigate to Configuration > Business Process. Highlight the BP that is going to hold the amount and click Open > Data Elements. You should see a Query tab. In the upper form, select the QBDE element and define any conditions, such as all records must be in approved status and they total on the Amount field.
Nice piece of functionality!