First log in to your Blogger Dashboard and go to Layout > Edit HTML. Now search for the following code in the box below.
<b:if cond="'data:post.includeAd'">
<data:adend/>
<data:adcode/>
<data:adstart/>
</b:if>
This is is code which show the ads when you enable 'Show ads between posts'. Cut this code and paste above this in your template.
<p><data:post.body/></p>
You may have to click on Expand Widget to see the code.
You can also tweak these codes more to show ads only on post pages and not when an user visits your home page, or browse categories. To do this you will have to change your code into this.
<b:if cond='data:blog.pageType=="item"'>
<b:if cond="'data:post.includeAd'">
<data:adend/>
<data:adcode/>
<data:adstart/>
</b:if>
</b:if>
If you use Blogger, then I suggest you try this.