there is a button on the Asset to Download HTML
Like Josh said, you can download the html in the Email Actions menu OR you can click Edit Code and copy the entire code and paste it to wherever you need to reference it (like Dreamweaver or something)
Like the guys said above. You can right click on an email in the marketing activies area and you will see the download HTML or you can also click on the drop down at the top once you have selected an email in the left hand menu.
Or when in the edit area of an email you can get to it again from the top menu.