Let me walk you through the process:
1. Click on "My Works"
2. Go to the work you wish to upload a cover for
3. On the left side there's the standard WP cover, right underneath, there's a little button that says "EDIT"** , click on it**
4. Either Create a new cover using the WP Cover Creator or click on Upload to upload one that already exists on your computer
5. Hit "Save"
Hope this helps everyone figure out how things work.