There is a very useful plugin is called Raw HTML Snippets.
This snippet allows you to add things like facebook widgets, google ads, and email subscribe forms to your sidebar or in a post.
You will see the plugin under Settings > Raw HTML Snippets.
Here's how to do it...
Create a new snippet and paste your ad code there.
Save the snippet and click the link that says "<<back to main page".
On the main snippet page, copy the shortcode including the brackets (ctl-c) into your buffer.
You can then paste this shortcode where ever you would normally paste the ad code - and now it won't get mangled!
Sample 1: In A Widget
Sample 2: In The Content Of A Post
If not, we will be happy to do this for you. Just open a ticket and send us you ad code and where you would like the ad to appear.