Select Insert -> Text from the main menu or press this button on the toolbar.
A text element will be inserted to the page.
To add text to it or edit the text in it, double-click on it to open it in the Text Element Editor. You can also right-click on it and select "Edit element" from the context menu to open it in the Text Element Editor.
Enter or edit text and then press the "Save and Close" button to save the changes.