cannot find a field for creating a condition table

Hi all,

I m trying to create a shipment scenarios which involves shipment cost based on prices,boxes,material....etc. to start with i tried to create a condition table, here the key combination needs to have the filed 'boxes' in it. I am not able to find it even in the master table. Is there any other place whr i can find it or how can i add this into the list?(master table)

any input is highly appreciated.

regards/aryan.