I haven't used Booksmart for quite a while - now prepare in InDesign. But from memory you can step through your book two pages at a time from within Booksmart but the preview function is properly available on the Blurb website after you have uploaded your book. So you can check that you have got the pages right from within Booksmart, then you upload and before ordering preview on the website to make sure that Blurb has received it as you intended.
As regards the text box, it may well be that you don't actually have a text box on the cover where you think you do. You can then edit the layout to include a text box. Again, from memory, I don't think you specifically have to choose a text box insert tool to insert text - merely need to click inside what Booksmart understands to be a text box on the relevant page/cover.
Good luck
