Thanks a lot for your help! ... I was able to get what I want.

private void Form1_Load(object sender, EventArgs e)
{
   DataTable table1 = new DataTable();
   table1.Columns.Add("Value", typeof(int));
   table1.Rows.Add(6);
   table1.Rows.Add(3);
   DataTable table2 = new DataTable();
   table2.Columns.Add("Tooltip", typeof(string));
   table2.Rows.Add("Apple");
   table2.Rows.Add("Bat");
   ultraChart1.Data.DataSource = table1;
   ultraChart1.Data.ZeroAligned = true;
   Hashtable labelHash = new Hashtable();
   labelHash.Add("CUSTOM", new MyLabelRenderer(table2));
   ultraChart1.LabelHash = labelHash;
   ultraChart1.Tooltips.FormatString = "<CUSTOM>";
}

   public string ToString(Hashtable context)
   {
      int row = (int)context["DATA_ROW"];
      int col = (int)context["DATA_COLUMN"];
      string customLabel = _table.Rows[row][col].ToString();
      return customLabel;
   }
}

Ok, I have a datatable, TABLE1, which I can bind to a column chart and the chart gets generated and it looks great. Now I have another data table, TABLE2 where I have a set of values which I have to show as tool tips for each of the columns/bars in the chart. Ex: Tool tip for the first column/bar in the chart should be 'Apple'. Tool Tip for second column/bar in the chart should be'bat' etc.  TABLE2 has these values, 'Apple', 'bat' etc.

Now can you give me a sample code to set the tooltip values from TABLE2?

Have you tried using the code in the link that i posted?
It's rather difficult to give you a specific example, since I don't know what exactly you mean by 'custom tooltips'. Can you explain what custom functionality you're looking for?

I have a column chart and I want to show custom tool tips.. can you please explain this with an example?