The criteria to use under the field would be IS NULL (note the space).
Field: WhateverField
Criteria: Is Null
The second question "Remove any duplicates" is more complex and you need to
post more information on what you want to do. As a start take a look at the
Query Wizard to create a FIND Duplicates query.
If that doesn't help you, then post back.
Remove any duplicates could mean DELETE the records with the duplicate
values or it could mean "Do not DISPLAY" the duplicate values. So the first
thing you want to do is to clear up what you are attempting to do.