Auto ID.

[RuralGuy]

Try this one.

[RuralGuy]

And to answer your next question, look at Query1 in this attachment.

[ssanfu]

The subform has the "Data entry" set to true. This setting will not allow you to see previously entered records. The "Data Entry property should be set to NO.

The OP shows two tables in the example database. Are the table structures the same for the real tables?

If there are common fields - like "FieldName" in the example tables - then you should be able to use an update query to update the "IDToUpdate" field.
The query I used is:

UPDATE tblToUpdate, tblSource SET tblToUpdate.IDToUpdate = [tblsource].[id]
WHERE (((tblSource.fieldname)=[tblToUpdate].[FieldName]));

Then you should look at normalizing your table structure.

"ID" is not a good name for a field, especially if used in multiple tables. Plus it is not very descriptive.
The autonumber should not be visible on forms, nor should the foreign key field. (I do make them visible during development to ensure the correct linking)

See: Autonumbers--What they are NOT and What They Are
http://www.utteraccess.com/wiki/index.php/Autonumbers