Hi. I have done this a few times before, so I'll give you a few tips.
As other people have sayed before me, the button after the sentence is the record button. You have to click it once to start, then click it again when you are done. There is no indication of you clicking it though. I recommend using headphones for this, in case you wanna read he sentence and pause with shift so you can read it as you are hearing it, if the sentence is too long to memorize. I do that, not sure if anyo e else does. And I hope that the recording device issue can be sorted out.
But after that, you go down to the table, and every time you finish the recording, there is a row on there that sais validating. If you are OK with what you recorded just now, just wait till that text sais ready instead of validating. Then you can continue with the next sentence. If you are not happy with your results and wanna do it again, you have to first click one of the unlabeled buttons on that row, it's probably the only unlabeled button. The way you know you clicked it is when you go back to where the sentence was, it goes back to the sentence you just recorded, so you can do it again, instead of the next sentence in line. Keep clicking buttons until that happens. Then, once the recording sais not valid, or any time when it sais not valid for that matter, the last button on the row is the delete button.
When you are playing back clips you have made with your voice, there is the play button before the text you entered to be spoken, then a share, download, and delete button, from what I remember. I could be wrong on that. You can't download your recordings, just the stuff you told it to say once your voice has been created.
I hope some of that helped.