you can find a master page control from content page like this
//your div should contain the property
runat="server"
//finding the div associated with id
System.Web.UI.HtmlControls.HtmlGenericControl currdiv = (System.Web.UI.HtmlControls.HtmlGenericControl)Master.FindControl("divid");
//hiding the div
currdiv.Style.Add("display", "none");