Selecting and installing an app must be done on the device itself, in my case my Surface tablet. This is unlike Google's Play Store for Android or Microsoft's own Web-based Windows Phone store which both allow you to shop for apps on the Web and send the app to a target device. This means that you start by launching Internet Explorer on the Surface tablet itself and pointing it at the Office Store and Apps site. Press the "Add" button after you find the app you want. However, note that there are a few more steps before you can actually use the Office app.
Next slide: First step - are you sure?