Measure with fields from two different tables

Hello All

I'm a really beginner user so . . . .

I have two tables as bellow:

Products [Prod_ID;Description;Prod_Weight]

Orders [Order_ID;Prod_Id;Quantity Manufacture]

I need to calculate the Total Weight in the Orders Table:

TW = Prod_Weight * Quantity Manufacture

Once I start to create a measure I'm not able to refer to field Quantity Manufacture in Products Table.

Where is my mistake?

Thank you in advance,

Elton Senne