This week's book giveaway is in the OCMJEA forum. We're giving away four copies of OCM Java EE 6 Enterprise Architect Exam Guide and have Paul Allen & Joseph Bambara 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.