The best place to get assistance about "how to" do something in PowerPoint is the PowerPoint newsgroup. You'll find a link in the forum's
Please Read First message. When you ask, be sure to mention which
version is involved.
Quite likely, the answer you'll get to "how do I..." will be in a VB language. But you will get the logic and the basic syntax. If you have difficulty "translating" VB to C# we can probably help you with that. But since the only relationship VSTO has to PowerPoint is providing the VS template for creating an add-in, there just isn't a lot of knowledge about the object model here in the VSTO forum.
The short answer to your question is (I think):
you get the text from a textframe2 using it's TextRange.Text property: theTextFrame2.TextRange.Text
Cindy Meister, VSTO/Word MVP