To add a layout code to your Ning page, click the "Customize" My Page link below your image.
At the top of this page it will say "Edit the Appearance of Your Page"
Scroll down past the theme choices to the area that says "Now, make it unique"
Here there are three tabs,
* Basic Theme
* All Options
To add your code you will want to click on "Advanced"
Once here you will likely see another code.
Just delete that code and paste in your new code.
Once you have your code pasted in there click on Save.