There are two ways to install lessons using Android Flashcards. The first (and easiest) is to use the in app browser. Alternatly you can copy the lesson .xml/.csv to your sdcard manually.
In App Download
When you are at the main screen you can hit the menu button and choose "Download New Lessons". This will bring up a list of available lessons and from there you can select the lessons to download. They will be automatically installed for you when you download them.
Manual Install Lessons are stored in the "flashcards" folder of your sdcard. The first time you run Android Flashcards, this folder is created. When you mount the card you will see this folder and you can put new lessons in it.
You can also create sub-folders in the flashcards folder for organizational purposes. These will show up in the lesson select list with a folder icon. A planned feature is in-app lesson organization