A white text box will appear; type your text inside of it. You can just use the Enter key for line breaks; I've broken mine up so that it'll fit inside my text box, and sized my font up to a size 20, bold, so that it's legible (I had it on 12, regular, and it was microscopic). Since I decided that Lex likes to race cars, he's going to talk about the big drag race coming up this weekend. (Humor me on this one.)
When you're done, just click on the Arrow tool to exit out of the text box; your text will appear normally (without the white text box) against your background, though a blue bounding box much like the one for symbols will show the outlines of your text area. Click and drag on your text to position it inside of your speech bubble appropriately.