Ok, just to make sure I'm trying to solve the right problem, you want the CallsScreenedMTD to increment by 1 each time any Employee listed in the tblEmployee table starts a new record in the frmScreeningForm? And then at the beginning of each month the CallsScreenedMTD is reset to 0?
Well I'm not sure about the query method, but the way I would do it it is to put the following vba code in the forms after update event.
dim strSQL as string
strSQL = "UPDATE tblEmployee SET tblEmployee.CallsScreenedMTD = [tblEmployee]![CallsScreenedMTD]+1 " _
& "WHERE (((tblEmployee.EmployeeName)=[Forms]![frmScreeningForm]![cboEmployee]));"
If you wish you can also stop the warning message appearing telling you that you are about the append records. But judging by what you've done so far with this database I feel you probably know how to do that.