Call an element from custom.bo to customer.xbo

Hi,

I need to call an element from DistributorPlanningForm.bo to Event beforeSave in customer.xbo. I have an alternativeKey CustomerID in my DistributorPlanningForm.bo. So i retrieve the CustomerID and call the element Annualized2015 and PurchasesYTD2015 into the calculation formula in the Event beforeSave - customer.xbo but i get the error as below. How to retrieve the elements? Kindly help.