void tabStrip_MouseClick(object sender, MouseEventArgs e)
{
    if (e.Button == MouseButtons.Right)
    {
        NDocumentPanel host = nDockManager1.DocumentManager.ActiveDocument.Host as NDocumentPanel;
        if (host != null)
        {
            NTabStrip tabStrip = host.Parent.Controls[0] as NTabStrip;
            if (tabStrip != null)
            {
                NContextMenu menu = new NContextMenu();
                //Add some commands.
                menu.Show(MousePosition);
                //To determine on which tab you have clicked you can use:
                NTab clickedTab = tabStrip.Tabs.TabFromPoint(e.Location);
            }
        }
    }
}