Master Page problems

try:

Button myButton = (Button) Master.FindControl("buttonId");
if(myButton != null)
{
    myButton .Visible =false;
}

The best way to do this would be to have a public property on the master page. this property can change the visibility of the controls and you can use this property from child pages conditionally.

see this example:
in my master page i have (RightSide is name of a Panel in my master page)

   public bool SidePaneVisibility
   {
   get
   {
    return RightSide.Visible;
   }
   set
   {
      RightSide.Visible = value;
   }
}

and I am using it from my content page conditionally as:

if (Session["UserId"] == null)
          {
              MasterPage master = (MasterPage)this.Master;
              master.SidePaneVisibility = false;
          }

If you need to understand the concepts then read this once: Beginner's Tutorial on Master Pages in ASP.NET[^]

Have a look at the following article. It shows you how to access the MasterPage from content pages:

http://msdn.microsoft.com/en-us/library/c8y19k6h.aspx[^]