WireframeApp uses font rendering of Google Font and because of that, text looks neat. The benefit of Google Font is that when you're creating a website design you're sure that text can be found online. WireframeApp also supports Text Styles so you can customize your typeface, size, color, etc.
You can add text by choosing the Label or Paragraph stencil tool. You can click anywhere on top of the stencil elements and the pointer changes to Text Input icon. When the text gets too big to fit inside the box, it will create a new line, as opposed to a normal text box which increases its width to fit the content.
When you resize a text box in the canvas its text size will not change. To change the text size, you must edit the text size value in the Format panel.