protected void DataList1_ItemCommand(object source, DataListCommandEventArgs e)
{
   foreach (Control var in DataList1.Controls)
   {
      CheckBox ch = (CheckBox)var.FindControl("chkItem");
      if (ch != null)
      {
           if (ch.Checked)
           {
               Label lb = (Label)var.FindControl("Label1");
               ListBox1.Items.Add(lb.Text);
           }
       }
    }
 }
 

<asp:DataList ID="DataList1" runat="server" OnItemCommand="DataList1_ItemCommand" Width="447px">
     <ItemTemplate>
        <table>
           <tr>
              <td>
                  <asp:CheckBox ID="chkItem" runat="server" /></td>
               <td>
                  <asp:Label ID="Label1" runat="server" Text='<%# Eval("CustomerID") %>'></asp:Label>
                </td>
              </tr>
          </table>
       </ItemTemplate>
  </asp:DataList>