You can create a new value for an attribute by entering it directly into the casebook.
To add a value in the casebook:
On the Tools menu, click Casebook.
Click the Open Casebook option.
The casebook is displayed in Detail View.
Click in the cell for the required attribute.
Click the drop-down arrow.
The value cell becomes editable.
Enter the new value
Click out of the cell.
The value is now available to be applied to other cases.