This is working here, triggered by an extra shape on the slide that runs this macro:
Sub TextBoxTotal()
Dim oShape1 As Shape, oShape2 As Shape, oShape3 As Shape, oShape4 As Shape, oShape5 As Shape, oShape6 As Shape, oShape7 As Shape
Dim OLENum As Integer, OLENum1 As Integer, OLENum2 As Integer, OLENum3 As Integer, OLENum4 As Integer, OLENum5 As Integer, OLENum6 As Integer, OLENum7 As Integer
Set oShape1 = ActivePresentation.Slides(4).Shapes("TextBox1")
OLENum1 = CInt(oShape1.OLEFormat.Object.Text)
Set oShape2 = ActivePresentation.Slides(4).Shapes("TextBox2")
OLENum2 = CInt(oShape2.OLEFormat.Object.Text)
Set oShape3 = ActivePresentation.Slides(4).Shapes("TextBox3")
OLENum3 = CInt(oShape3.OLEFormat.Object.Text)
Set oShape4 = ActivePresentation.Slides(4).Shapes("TextBox4")
OLENum4 = CInt(oShape4.OLEFormat.Object.Text)
Set oShape5 = ActivePresentation.Slides(4).Shapes("TextBox5")
OLENum5 = CInt(oShape5.OLEFormat.Object.Text)
Set oShape6 = ActivePresentation.Slides(4).Shapes("TextBox6")
OLENum6 = CInt(oShape6.OLEFormat.Object.Text)
Set oShape7 = ActivePresentation.Slides(4).Shapes("TextBox7")
OLENum7 = CInt(oShape7.OLEFormat.Object.Text)
ActivePresentation.Slides(4).Shapes("TextBox8").OLEFormat.Object.Text = OLENum1 + OLENum2 + OLENum3 + OLENum4 + OLENum5 + OLENum6 + OLENum7
End Sub