Sorry, I should have noticed before, the correct reference to cell F29 in VBA
is Range("F29") or Cells(29,6). Check the format of F29, it should be Date!

Sub ReminderMessageOnOpening()

Dim ReportType As String

If ThisWorkbook.Sheets("Instructions").Range("C29") < "" Then
ReportType = ThisWorkbook.Sheets("Instructions").Range("C29")
If ThisWorkbook.Sheets("Instructions").Range("F29") = Date Then
MsgBox ("Reminder.... Project " & ReportType & "due today")
End If
End If
End Sub

Regards,
Stefi