The relative path parameter for the addNode() method shouldn't start with a "/". Try
Node user = videojetNode.addNode(username);
Though i agree that the term "relPath" in the docs is quite misleading, the relPath should either be the name of the node you would like to create under the current node or should start with the name of the child node and contains the relative path to the target node under which you want to create your node.
For example. In case the current node is content and you have the following tree
/
|_content
|_x
|_y
and if you wish to add a node called z as a child of y, then the relPath can be specified as
Node myNode = contentNode.addNode("x/y/z");
Note : A PathNotFoundException
will be thrown in case any of the intermediary nodes are not available