To add text inside an existing shape:
Select any of the Pointer Tool, Text Tool, Reshaping Tool or Rotation Tool.
Double click on a shape to open the column container for text input and insert the text cursor. The Text Tool is auto-selected, ready for text input.
You can now add text by typing or pasting it in.
As an alternative:
You can drag text from elsewhere in your iStudio Publisher document, or from other applications, and drop it into a shape.