After you've logged into Twitter, go to their widget page and click on 'Create New'.
Choose 'Profile' from the dropdown menu, and select 'Embedded Timeline'.
Type the URL of your Twitter page, e.g. http://twitter.com/moonfruit.
From here, you can customise your feed widget including the size and theme.
Click 'Preview', then copy the code above the preview displayed.
Navigate to your Moonfruit site editor. Click the 'Insert' tab in the main toolbar > 'HTML snippet' in the left sidebar.
In the floating editor, click the 'Setup' tab, paste your code, then Apply and Save your changes. You can review the feed by clicking 'View my site' in the top right corner. NB: You may need to resize the widget in the editor to avoid part of the widget being partially cropped.