Julie,
Do you have another allocation line that you are spreading up to applications? If so the way that I have done this in the past is the following:
Within the model I'd go to the Applications object and do "Show\Hide Columns" and type in driver. Then I'd select all the "Cost" drivers that are coming into that object.
I would export this for ease of copying and building the metric that you'd want to have calculated. The brackets have to be around each of the drivers.
In this example I am taking two of the service allocations for the cost drivers of Indirect_1 and Direct and adding them together in order to weight my third allocation by this amount (at the BU level).
Once you have created the metric you can draw the allocation line on the model and use the "Weight By" selection to choose your newly created metric.

Cheers,
Ryan