To add a text box to the page, click on the Text palette on the left vertical toolbar.
From there, click on the Text tool.
This will change your cursor to the Text tool. Then, click and draw a box on the page for how big you want the text box to be.
This will add a text box to your page and you can immediately begin typing. Note that after you add the text box to the page, the cursor will immediately turn back into the normal cursor. If you wish to add multiple text boxes at once, double-click on the Text tool. Then, after you add a single text box, you will be able to add another one without having to click on the Text tool again. Single-click on the Text tool once more to get the cursor out of Text tool mode or click on an image on your page, both have the same effect.