Simple Checkbox manipulation

I am trying to use JS to set the value of the isinactive checkbox equal to checked or T when a certain status is selected.  However I cannot seem to figure out how to call the checkbox.  I have to examples of how I have tried to call the checkbox and set the field value.  Neither works.  Little help?

function OnFieldChanged(type, name)

{

//This function checks for Status Fields and then throws alerts and changes status

var NewStatus = nlapiGetFieldValue('custentity_contactstatus');
var NewActive = nlapiGetFieldValue('isinactive');
var SetStatus = 'Y';

//alert(NewStatus);

//alert(NewActive);

if(NewStatus == '8')
  {
   alert('Please edit the Marketing Event tab to qualify the contact on this Marketing Event.');