Like the JContextTree, the JContextTreePopup is a frame class. It's not a JPopupMenu (well.. yes at the end it will be a JPopupMenu, but you cannot access it directly). It's not just a popup menu, this one is dynamic, what he shows depends on what is selected in the tree.
How to use it ?
The JContextTreePopup works with TreePopupItem.
Each TreePopupItem is a component that might be visible in the final JPopupMenu but only if the condition described in the TreePopupItem are met.