I think you just go to Google or Amazon (or other) store and download what you want again. Amazon does store your "purchases" and they are in the Cloud. I don't think Google does anything like that but I'm still rather new to Android and Google. If you did purchase it, then you need to setup the Tab with that gmail address.
If I understand the second question, Press the "button" to the left of the Home button. If you have an app opened, its Menu Button might be at the top right or for Amazon store, top left and even in the other corners.