ActiveWorkbook means nothing to PowerPoint (unless you set an excel reference.) Also I would close Excel before the message box.
Private Sub RetrieveExcelValue()
Dim xlApp As Object
Dim xlWorkBook As Object
Dim ExcelValue As Double
Set xlApp = CreateObject("Excel.Application")
xlApp.Visible = True
Set xlWorkBook = xlApp.Workbooks.Open("E:\Special Projects\PPT Project for Matthew Orenchuk\Cross Flow Skid EER.XLSM", True, False)
ExcelValue = xlWorkBook.Worksheets("PPT Transfer").Range("C5").Value
xlWorkBook.Save ' probably not needed
xlWorkBook.Close
xlApp.Quit
MsgBox ("The value of this range in Excel is " & ExcelValue)
End Sub