in the query that your report is based on, add a calculated field using whatever it is that you use to calculate 'overdue', and if the record is OVERDUE, set this calculated field to 1, otherwise set it to 0. then at the report footer, simply add this field: something like this:
Overdue: iif(now()>[DueDate],1,0)
where [DueDate] is your due date field. in any case, make this calculated field in your query that the report is based on. then in the report design, bring that field into the detail section with the rest of each record. you can make it invisible.
then in the report footer, put a text box and in it put
=sum(Overdue)