Step 1#.
Login to your blogger account and click on Design >> Edit HTML >> Mark as Expand Widget Template
Step 2#.
Search for the code
<div class="post-body entry-content">
For searching the code press "Ctrl+F"
Step 3#.
After getting the above code,you have to paste the following codes below that.
<div style="float: left; margin-left: 5px; margin-right: 10px;"> <b:if cond="data:blog.pageType == "item""> YOUR PARSED AD CODE </b:if></div>
Make "YOUR PARSED AD CODE"
Step 4#.
After these steps save your template and you are done with it.
NOTE:
If you want to align your ad left,right or center, just change the code from
<div style="float: left; margin-left: 5px; margin-right: 10px;">
"float: left" to "float: right" or "float: center"
You must parse your ad code, otherwise it will not work.