Good Morning.

Welcome to the Forum. It's always good to have new members. Just in case you didn't realize, from the main VBA Express Forum page, you can select a variety of sub-Forums that relate specifically to a particular Application. In order to get the best answer possible, please post your questions to the appropriate Forum.

That being said, for your specific question, you will need to use a Macro Action. Inside the Procedure, you'll want to access the particular TextBox using:

Ensure Macros are enabled in PowerPoint.
Press Alt+F11 to open the Visual Basic Environment.
Right-click the Project Explorer and Insert Module.
Create a new Public Sub.
Close the Visual Basic Environment.
Insert your TextBox.
Right-click the TextBox and select Custom Action.
Set it to Run Macro.

[VBA]
ActivePresentation.Slides(n).Shapes(n).TextFrame.TextRange.Text = n
[/VBA]

Hope this helps.
Scott