Hope im not digging up an old thread but this is rather easy to fix. Just open up phpMYadmin and expand your prestashop database. Mine would be labeled as "pr." Inside that is a lot of tables called ps_
Look for: ps_cms_lang
And all your CMS pages will load. Click on edit on the page you want to add the HTML code to. Paste in the code to the big box at the bottom. And then when your done scroll down and make sure save is selected from the drop down menu and click on go.
Viola you just bypassed prestashop
Thank me later