SELECT TBL1.*, TBL2.*
FROM TBL1 LEFT JOIN TBL2
ON TBL1.KeyField = TBL2.KeyField
WHERE TBL2.KeyField Is Null
Replace KeyField with your column name.
The LEFT JOIN includes all records from TBL1, and joins all records from TBL2 that have a match, leaving those w/o a match with null values in the TBL2 columns. The WHERE clause then takes out the rows that successfully joined.
Viewing the returned recordset without the Where clause may be more helpful in trying to understand how the tables join.
You may want to try this, as well:
SELECT TBL1.*, TBL2.*
FROM TBL2 LEFT JOIN TBL1
ON TBL2.KeyField = TBL1.KeyField
WHERE TBL1.KeyField Is Null
PS You can enforce referencial integrity among tables by creating relationships under Tools|Relationships.