Need Help using FlowField

My client needs to see their item sales and purchases according to specific salesperson and principal. So we made Global Dimension 1 Code as Principal Code and Global Dimension 2 Code as Salesperson Code.
Then, I made a form with Table 27 (Item) as Source Table and put Global Dimension 1 Filter, Global Dimension 2 Filter, and Date Filter controls. I hoped when I change the value in those controls, the Purchase (Qty.) and other fields that alike will also change accordingly.
It works fine when I typed in Date Filter control , but when I put a value in Global Dimension 1 Filter or Global Dimension 2 Filter control, this error message showed up

[color=blue]The FlowField value cannot be calculated

    You must define and activate a key that contains the key fields that  appear below and then associate the specified SumIndexFiled with it

   Key Fields : Global Dimension 1 Code, Item No., Entry Type
   SumIndexField : Invoiced Quantity
   Table : Item Ledger Entry[/color]

I know that Item Ledger Entry table has Keys containing Global Dimension 1 Code, Item No., and Entry Type. I just couldn't find out where should I put a code to activate the key. I tried to put

ItemLedgEntry.SETCURRENTKEY("Global Dimension 1 Code",...)

in OpenForm, OnGetCurrRecord, OnAfterGetCurrRecord triggers, also in Control -Validate and AfterValidate, but it didn't work

Help me please