The following example shows how you can use the itemDoubleClick event to open the selected Tree node in Flex.

<?xml version="1.0" encoding="utf-8"?>
<!-- http://blog.flexexamples.com/2007/11/29/using-the-itemdoubleclick-event-to-open-nodes-in-a-flex-tree-control/ -->
<mx:Application xmlns:mx="http://www.adobe.com/2006/mxml"
        layout
="horizontal"
        verticalAlign
="middle"
        backgroundColor
="white"
        creationComplete
="init();">

    
<mx:Script>
        
<![CDATA[
            import mx.events.ListEvent;

            private function tree_itemDoubleClick(evt:ListEvent):void {
                var node:XML = tree.selectedItem as XML;
                var isOpen:Boolean = tree.isItemOpen(node);
                tree.expandItem(node, !isOpen);
            }
        
]]>
    
</mx:Script>

    
<mx:XMLListCollection id="xmlListColl">
        
<mx:source>
            
<mx:XMLList>
                
<node label="One">
                    
<node label="One.1">
                        
<node label="One.1.1">
                            
<node label="One.1.1.1">
                                
<node label="One.1.1.1.1" />
                            
</node>
                        
</node>
                    
</node>
                    
<node label="One.2" />
                    
<node label="One.3" />
                
</node>
                
<node label="Two">
                    
<node label="Two.1" />
                    
<node label="Two.2" />
                    
<node label="Two.3" />
                
</node>
                
<node label="Three" />
            
</mx:XMLList>
        
</mx:source>
    
</mx:XMLListCollection>

    
<mx:ApplicationControlBar dock="true">
        
<mx:CheckBox id="checkBox"
                label
="doubleClickEnabled:"
                labelPlacement
="left"
                selected
="true" />
    
</mx:ApplicationControlBar>

    
<mx:Tree id="tree"
            dataProvider
="{xmlListColl}"
            labelField
="@label"
            width
="250"
            rowCount
="6"
            doubleClickEnabled
="{checkBox.selected}"
            itemDoubleClick
="tree_itemDoubleClick(event);"
            showScrollTips
="true" />

</mx:Application>