Hello,
As the steps mentioned in the link provided by you I believe that Code needs to be inserted in to the Head section of the page.
To insert any code in to the head section of the page you need to go to following location :
1. Open the page (design Mode) that need to have the "Smart app banner"
2. Go to page > Page properties > MetaData > HTML for <head>
3. Paste the code there and click OK.
Commonly inserted code would be like <meta name="apple-itunes-app" content="app-id=xxxxxxxxx"/> where xxxxxxxxx need to be replaced by your app ID.
There are few more details that you need to keep in mind. I would refer you visit to the link below and read the instructions below "Getting Started" and "Showing an Smart App Banner in Plain HTML"
http://www.raywenderlich.com/80347/smart-app-banners-tutorial
Regards
Vivek