Your javascript code is not valid, it can't be parsed by the browser.
You are asking to show some part of the page when you are in the page "yourbloghost.com/b/post-preview?token=something", right?
You can do this with something like:
$(document).ready(function(){
if (window.location.pathname == "/b/post-preview")
{
document.getElementById('Main-Section').style.display = 'block';
}
});
or also:
$(document).ready(function(){
if (window.location.pathname == "/b/post-preview")
{
$("#Main-Section").show();
}
});