So I have tried several ways to do this, none are working.

We have a shipment table in a Subform.  The aim of the table to manage part shipments etc.

What my management want is the total sum of shipments that have gone to be calculated.  The solution I had was to add a checkbox, so we can mark the shipment as gone, However when I try and build the formula I am not able to see the check box in the fields I can use.  I was going to simply say if the checkbox is true sum it, but it does not want to play... 

I also tried adding a subform formula adding a column, but again I can not sum based on if the check box is true.  

its probably going to be glaringly obvious, but its evading me at the moment.  Any thoughts?