First, make sure you're logged into your account at https://marketplace.hmhco.com.
In the top navigation menu, you'll see your name. Click it to view a drop down menu.
From the drop down, select "View your Past Purchases."
You'll be taken to a list of the items you've already purchased. From the list, you can re-download files you've purchased, or click to find apps you've accessed in the past.