Always post support questions to Products (I don't know where this post was before, but a moderator just moved it today).
You'll have to explain more about what isn't working with the embed code for you. How exactly are you adding the (HTML + JS) ingredients of the embed code to your site?
You can embed a frame into a landing page in marketo and use that landing page as an Iframe. the only things you should note is that unless you add additional HTML code upon completing the form it will open the other landing page or file in that iframe. Additionally if you have an HTTPS site this could be more problematic as some people may receive errors that the Iframe is not protected. I would highly recommend working with your web developer to adding in the HTML embed code to the page, there may be something as easy as clicking text in the visual editor that you may have missed.