You are missing these two namespaces
using System.Web.UI;
using System.Web.UI.WebControls;
if you are using ASP.NET
try this code hope it will help.
using System;
using System.Collections.Generic;
using System.Linq;
using System.Web;
using System.Web.UI;
using System.Web.UI.WebControls;
public void loadSuppliers(DropDownList ddl)
{
}