Join Date:Joined: Jun 2010
This is the wrong place to ask as this is the development section, your question should be posted in XPERIA X10 General section. (I guess mod will move this topic soon).
To get the key for Documents To Go you need to buy the application on the android marktet (it's US$ 14,99), but that unfortunately can only be done if you have a credit card, google doesn't offer any other payment options.
I don't know if there is any free app that will do the job, but odds are there is.