when you've embedded it, you've specified the width of the iframe to be 700 pixels. find the bit that says width="700" change that to width="100%" and you should be fine.
The code block does have a bit of padding on it which you probably don't want in your mobile version as this will probably upset the way the form fills the width, so you might also want to add this code to your code block (put it above the iframe bit and you'll be fine, if not add it to the custom css part of your site from the main Design menu without the style tags around it).
<style>
@media screen and (max-width: 767px) {
#block-yui_3_17_2_1_1628417923165_5434 {
padding:0;
}
}
</style>
