Try this instead:

Sub Calulated_Average1()

Dim oRangeSelected As Range
Dim MyAverageIs As Double
Dim dblTotal As Double
Dim intCellCount As Integer
Dim c As Object

'On Error Resume Next
Set oRangeSelected = Application.InputBox("Please select a range of cells!", _
"SelectARAnge Demo", Selection.Address, , , , , 8)
If oRangeSelected Is Nothing Then
    MsgBox "It appears as if you pressed cancel!"
Else
    For Each c In oRangeSelected.Cells
        dblTotal = dblTotal + CDbl(c.Value)
        intCellCount = intCellCount + 1
    Next c
    ActiveCell.Value = dblTotal / intCellCount
    'message box is not needed but may help you
    MsgBox "You selected: " & oRangeSelected.Address(External:=True)
End If

End Sub