Ribbon Workbench mscrm2011

Hi,

Based on my field selection in case form i need to disable the custom buttons which i have created using ribbon workbench.

 here i am taking some custom  rule and by using that rule i have created one webresource with function name nd using that i am calling that function in enable rules in workbench.

My Jscript code as

function refreshRibbonOnChange()
{
    Xrm.Page.ui.refreshRibbon();
}

function enableButton()
{
var formType = Xrm.Page.ui.getFormType();
 
           if (formType != 1) {
  var attribute = Xrm.Page.getAttribute("fmc_casenumber");
  if (attribute!=null)
{
return (attribute.getValue()==true);
}
else
{
return true;
}
}
}

i have tried with this code but it is not wrkng

Can u plz help me out

Regards,

hiteksha