If the field you wish for is not in the table related to you report - you can add the table yourself.
See 6. Wizard query for adding a table.
How do i know what table and what field name?
Right click in the field - choose Form information - Form Name
A new form pops up and under Administration you can see the table and field name
Stand on the field - right click
Choose - Personalize
The table and field name is now shown: (SalesTable).PurchOrderFormNum
How do I know which fields relate the tables?
Go to AOT (Ctrl+D)
Find table - Here InventTable
Open the table - open relations - find the table you want to connect to - open this - the relations are now shown