You can handle MouseDown, use GetNodeFromPoint to get the node at the cursor location (remember to check for null first), and set the node's Selected property to true.

Example:
private void ultraTree1_MouseDown(object sender, MouseEventArgs e)
{
    if ( e.Button == MouseButtons.Right )
    {
        UltraTree treeControl = sender as UltraTree;
        UltraTreeNode nodeAtPoint = treeControl.GetNodeFromPoint( e.Location );

        if ( nodeAtPoint != null )
        {
            nodeAtPoint.Selected = true;
        }
    }
}