When typing to add a reply or when creating a new thread; Just under the text box you will see "Additional Options." In the middle of Additional options is a "Manage Attachments" button, click on that and an pop up window will appear, in the window towards the top will be a "Browse" button. You should know what to do from here: once your done, don't for get to hit upload; and make sure you wait until your upload has finished before closing the window. Then just hit Submit Reply like normal.