Well, may be you want something like :
List<Int32> lstindex = new List<Int32>();
String[] splt = txtSelect.Text.Split(',');
// initialize list of indexed for textbox
foreach (String str in splt)
{
lstindex.Add(Convert.ToInt32(str) - 1);
}
// for each menu
foreach (ToolStripMenuItem mnItem in msMenus.Items)
{
// for each menu item
foreach (ToolStripItem item in mnItem.DropDown.Items)
{
// if index of item is in the list of indexed, set visible to false, otherwise to true
item.Visible = lstindex.Contains(mnItem.DropDown.Items.IndexOf(item)) ? false : true;
}
}