In order to upload a presentation or do anything on the whiteboard you must have presenter privileges. This is the icon to the left of your name in the user menu.
You can see what it looks like below. If you do not have this next to your name you don't have presenter privileges and you must click on the icon next to your name to give yourself privileges.
If you don't have the little person icon, which you can see next to 5 people's names below, it means you are in a student account and need to login with an instructor account.