The Nominal Code will need to be entered on the Elements Tab for the Part.
To do this you will need to following the below
Click on Parts
Locate the Part
Click on the Elements Tab
Right Click on the bottom of the table and click on Modify Record
Right Click again and click on Create Record
Enter in the Account Details
Click Apply at the lower right of the screen.
Once this has been completed, the Nominal Account should show when processing an order.