some help with workflow

Hey all,

I'm trying to customize a workflow and I need help.

I need workflow with 1 step that change a field to a mandatory field.

the workflow initiate while I want to receive an order and I open the item receipt page.

for inventory items I created a new field (check box) and I need the system to check if this box is checked, and if it is, i want to change a field in the item receipt to mandatory.

my problem is that I cannot find inventory item into the list under the state's formula screen (I can see discount item but inventory item I cant).