Just to be 100% sure we set the criteria correctly, enter the criteria as this way:
[yourColumnName]=[pOrg] OR [pOrg] IS NULL
When you save the query and open it, it will look a bit different because as explained in the book (pg. 244-246), Access query designer changes the expression into two columns. For multiple parameters, it could be daunting and thus, keeping the original criteria in a notepad is probably good idea when you need to edit the query.