In order to find your list subscription form, you have to navigate to your customer area, then from the left side menu, expand the lists menu and click on the Lists link. Doing this, will reveal a page looking like:
Choose the list you want to get the subscribe form for by clicking on the list name or on the list unique id, which will reveal a page like:
From here, if you click on the ‘Forms’ box, you will get the html/iframe code for embedding the form in your site, like:
And if you go to Quick links > List Pages, you’ll be able to see the subscription page and the link to it:
So it’s up to you if you want to embed the html form, the iframe code, or you want to use the direct link to the subscription form.