I jumped on this one. I wish it was about 6 months sooner though. Documents to Go was the first App I bought for my Inspire (I'm an old Palm user, so I went with the familiar name). So far I think I like Office Suite a little better.
I know, how awesome, right? This is the best free app of the day since Swiftkey, IMO. I wish they would have worthwhile ones more often. It's like 30 days of games and crap, then a decent non-game. But I shouldn't complain about free apps
Basically you need to have the unknown sources checked off. This is visible on Gingerbread. There are ways to enable it on Froyo. Do a search if you do not have Gingerbread. To check it off go to settings/applications. Should be at the top.
Then when you go to get the free app you will have do download the amazon app store app first. Run the apk file that downloads. This will install the app. From there find the free app of the day and download it. You will need to sign into your amazon account or create a new one and put in a payment option still even though the item is free. You do not get charged for free apps.