Here's some sample code that'll do what you want.
Rather than make it specific to your question, I wrote something more general-purpose that you can use to find any shape at any position on any slide you like.
Option Explicit
Sub AddTestTextbox()
' Use this to quickly add a text box to test with
' at the right coordinates
Dim oSh As Shape
With ActivePresentation.Slides(1)
Set oSh = .Shapes.AddTextbox(msoTextOrientationHorizontal, 300, 100, 200, 100)
oSh.TextFrame.TextRange.Text = "Some random text to test with"
End With
End Sub
Sub Test()
Dim oSh As Shape
Set oSh = FindShapeAt(ActivePresentation.Slides(1), 300, 100)
If Not oSh Is Nothing Then
If oSh.HasTextFrame Then
If oSh.TextFrame.HasText Then
MsgBox oSh.TextFrame.TextRange.Text
End If
End If
End If
End Sub
Function FindShapeAt(oSl As Slide, sngLeft As Single, sngTop As Single) As Shape
Dim oSh As Shape
For Each oSh In oSl.Shapes
If oSh.Left = sngLeft Then
If oSh.Top = sngTop Then
Set FindShapeAt = oSh
Exit Function
End If
End If
Next
End Function