To add text to the canvas (background), simply click the "Add Text" button on the left and type your text into the text field in the Text Properties tool palette that opens. If you don't see the text on the canvas, please make sure that the text is a different color than the canvas or background element.
To edit the text in a template, simply click any template in an application and select the text in the design by clicking directly on it, or by selecting it's layer in the Design Layers palette.
Once selected, miscellaneous tool palettes will open.
In the Text Properties palette, you can edit the text directly in the main text field. You can also change the font color by clicking the color picker, Change the opacity (transparency), adjust the spacing between letters, and if you have multiple lines of text, you can adjust the space between lines of text as well.
The Bold, Italics, and Underline will only work on regular fonts. You can also change the text justification similar to the same functionality in a word processor.
It is a good idea to test the functionality so that you are familiar with it but it is very straightforward.