Put the DSum value in an Aggregate Query and sort in there

You can only sort by a calculated field if the calculated field is part of the report's record source.  So that will be your goal: put the count into the record source.

You need two queries for this.  First make an aggregate query just for counting the notes:

     SELECT FullNameID, Count(Notes) As CountOfNotes FROM MyTable GROUP BY FullNameID
    
Name this query, say, NoteCountQ.  It will look like:

FullNameID     CountOfNotes
1000           5
1001           2
1002           4
1003           6

Then you join NoteCountQ with your main table:

    SELECT FullNameID, Notes, CountOfNotes FROM MyTable
    INNER JOIN NoteCountQ
    ON (MyTable.FullNameID = NoteCountQ.FullNameID)


which will look like:

FullNameID     CountOfNotes    Notes
1000           5               Note #1 of person# 1000
1000           5               Note #2 of person# 1000
1000           5               Note #3 of person# 1000
1000           5               Note #4 of person# 1000
1000           5               Note #5 of person# 1000
1001           2               Note #1 of person# 1001
1001           2               Note #2 of person# 1001
1002           4               Note #1 of person# 1002
1002           4               Note #2 of person# 1002
1002           4               Note #3 of person# 1002
1002           4               Note #4 of person# 1002
Etc.

Essentially, each person's count appears repeatedly for all his or her notes.  Use this as the record source of your report.  With CountOfNotes part of the record source, you will now be able to sort with it.