You can use any shape (except a line) from the Shapes menu as a text box.
To add a shape to a page, select the page by clicking it in the sidebar on the left of the iWeb window.
Choose a shape from the Shapes pop-up menu at the bottom of the iWeb window.
The shape appears with its selection handles showing.
With the shape still selected, type your text.
If there is more text than the shape can display, a “clipping indicator” appears. To display all of the text, resize the shape.