Codesnippet for creating an tree store from flat data:

var me = this,
    treeStore = me.getStore(),
    rootNode = treeStore.getRootNode(),
    node;

Ext.iterate(data, function(key, value){
    //get node for appending next node
    if(value.parent == 'root'){
        node = rootNode;
    } else{
        node = rootNode.findChild('id', value.parent, true);
    }

    // create and add new node
    if(Ext.isDefined(node) && node !== null){
        node.appendChild({
            id:key, 
            text: value.title,
            leaf: false,
            expanded: true
        });
        node.leaf = true;
        node.expanded = true;
    }
});