You can setup the tree for example that will display 2 levels.

<ig:WebDataTree ID="WebDataTree1" runat="server" 
    Height="300px" Width="200px">
    <Nodes>
        <ig:DataTreeNode Text="Root Node 1">
            <Nodes>
                <ig:DataTreeNode Text="Child Node">
                </ig:DataTreeNode>
                <ig:DataTreeNode Text="Child Node">
                </ig:DataTreeNode>
            </Nodes>
        </ig:DataTreeNode>
        <ig:DataTreeNode Text="Root Node 2">
            <Nodes>
                <ig:DataTreeNode Text="Child Node">
                </ig:DataTreeNode>
                <ig:DataTreeNode Text="Child Node">
                </ig:DataTreeNode>
                <ig:DataTreeNode Text="Child Node">
                </ig:DataTreeNode>
            </Nodes>
        </ig:DataTreeNode>
        <ig:DataTreeNode Text="Root Node 3">
            <Nodes>
                <ig:DataTreeNode Key="44" Text="Child Node 44">
                </ig:DataTreeNode>
                <ig:DataTreeNode Text="Child Node">
                </ig:DataTreeNode>
            </Nodes>
        </ig:DataTreeNode>
    </Nodes>
</ig:WebDataTree>

Under "Root Node 3" contains a child node with the key "44". To walk through every level of the tree you can create a recursive function.

Public Sub FindNodeByKey(ByRef findNode As DataTreeNode, ByVal node As DataTreeNode, ByVal key As String)
    For Each childNode As DataTreeNode In node.Nodes
        If childNode.Key = key Then
            findNode = childNode
        End If
        For Each nextChildNode As DataTreeNode In childNode.Nodes
            If nextChildNode.Key = key Then
                findNode = nextChildNode
            End If
            FindNodeByKey(findNode, nextChildNode, key)
        Next
    Next
End Sub

Within the recursive function will call the FindNodeByKey() method to walk through the next level if there are any child nodes.

At the beginning you can loop through the root nodes and then call FindNodeByKey() to walk through every child level.

Protected Sub Page_Load(ByVal sender As Object, ByVal e As System.EventArgs) Handles Me.Load
    Dim findNode As DataTreeNode = Nothing
    For Each node As DataTreeNode In WebDataTree1.Nodes
        Dim key As String = "44"
        If node.Key = key Then
            findNode = node
        End If
        FindNodeByKey(findNode, node, key)
    Next
    If Not IsNothing(findNode) Then
        findNode.Visible = False
    End If
End Sub

In the sample if there's a node (either root or child) that contains the key "44" then it will be hidden in the tree.