This week's book giveaway is in the OO, Patterns, UML and Refactoring forum. We're giving away four copies of Refactoring for Software Design Smells: Managing Technical Debt and have Girish Suryanarayana, Ganesh Samarthyam & Tushar Sharma on-line! See this thread for details.
I assume you know how to read all entries in a Zip file.
I would keep a mapping from entry names to DefaultMutableTreeNodes. Then, when you encounter a ZipEntry, you determine its parent from the name (using the / as the separator*). You retrieve the parent node from the map, creating it if necessary. Then you create a DefaultMutableTreeNode, and add it to the parent node. Finally you add the new mapping.