The standard way to embed js or html or gadget or widget is through add gadget features. This will show the gadget on entirely blog of your. In some cases we need to just show specific gadget on single blog post. How ?
Here is the trick.
Before changing settings of your template first save a copy of it by downloading it.
Go to Layout tab -> Edit HTML - > search for for ]]></b:skin>

Now paste the following code above ]]></b:skin>
<pre>
{
background:#efefef;
border:1px solid #A6B0BF;
font-size:120%;
line-height:100%;
overflow:auto;
padding:0px;
color:#000000 }
pre:hover {
border:1px solid #efefef;
}
code {
font-size:120%;
text-align:left;
margin:0;padding:0;
color: #000000;}
.clear { clear:both;
overflow:hidden;
}
</pre>
Now click on Save Template.That's it you are Done.
How to use this display boxEverytime when you want to display HTML or javascript inside your blog post, place the code in between the opening and closing "pre" tags
<pre>Your Html or Java script</pre>