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.
If you are using or extending javax.swing.tree.DefaultTreeModel, invoke fireTreeNodesInserted() on it. That will tell the model that the data has changed and it will fire events off to inform interested parties (i.e. the display). Look at the documentation for other useful event-firing methods.