Next time, please stick to your original thread for clarifications.
That said, uploading can be done using the link tool from the toolbar (first row, second from right). Write something like "sample GraphML", select it, click the link tool, go to tab "Upload", choose the file, go to tab "Link Info", click "Ok".
The error says "Caused by: org.xml.sax.SAXParseException: Character reference "" is an invalid XML character." which means you managed to enter "bad" text somewhere.