Javascript functin to add elements to a drop down list or options. 

function addOption(ddl,text,value )
{
var optn = document.createElement("OPTION");
optn.text = text;
optn.value = value;
ddl.options.add(optn);
}

This is how u call the function 
addOption(document.getElementById("<%=ddlCity.ClientID%>"),CITYNAME,CITYCODE);