To upload content to a Module, you must click on the "Add and Arrange Content" button, however, if you do not see it on the Content tab, it's likely due to the size or zoom level of your browser (Chrome, Firefox, IE, Safari).
When using a web browser, the width needs to be larger than 1024 pixels for the "Add and Arrange Content" button to appear. Depending on your web browser, you will need to expand the browser or zoom-out to "Actual Size".
Once you have expanded browser (ex. full screen) or have set the browser to "Actual Size", hit the refresh button (reload button) and the button will reappear.