Customizing data model in BI Publisher (Cost Detail Data Model)

Summary:

I would like to add some fields to the data model "Cost Detail Data Model" in Assets Balances.

Content (required):

I have copied the file to the custom folder, and just tried to create some very minor changes to the Q_Main dataset, just to see if I can do anything to it... But when I try to create a new line with the same data (asset_number) but just renamed, and press OK, it says "please enter values for lexical references in SQL-Q_MAIN... What am I supposed to put in here?

I'm trying to follow this guide