Did the library Reference for Excel reset when you opened the project in 2010? For 2010 it changed from 12 to 14.
I successfully tested creating textbox and setting value with simple procedure in Excel2010 VBA:
Code:
Sub AddTextBox()
ActiveSheet.Shapes.AddTextBox(msoTextOrientationHorizontal, 2.5, 1.5, _
116, 145).TextFrame.Characters.Text = "This is a test of inserting a text box to a sheet and adding some text."
With Selection.Characters(Start:=1, Length:=216).Font
.Name = "Arial"
.FontStyle = "Regular"
.Size = 10
.Strikethrough = False
.Superscript = False
.Subscript = False
.OutlineFont = False
.Shadow = False
.Underline = xlUnderlineStyleNone
.ColorIndex = xlAutomatic
End With
End Sub
So I know the Characters method works. Perhaps you need to start with simpler code that works then build it up.
Could you put the code behind the Excel book?