When you have finished designing your form you will see a piece of code you can copy. Highlight and copy this code.
Now login to your MoonFruit account and edit your website.
Click on the INSERT tool at the top of the page
Click on the HTML SNIPPET tool on the left hand menu.
You will see a new HTML Snippet box appear on your website. Drag the box around the stage until you get it to where you want the email signup form to appear.
On the little Editor Window (Normally located top-right) click the HTML tab.
Paste the code you copied in step 2 into the box and click APPLY...