the Materials editing dialog shown below comes up.

double-click the last row in the Materials column. A popup appears with the names of all materials.

select the desired material and press the TAB key to finish the edit.

the Estimated cost/unit value is filled in automatically. You can edit this value by double clicking it if required.

double click the Estimated units column to enter the number of units required.

click the Save changes button to save your edits.